God alone can give us right and full knowledge of this reality by revealing Himself as Father, Son and Holy Spirit, in whose eternal life we are by grace called to share, here below in the obscurity of faith and after death in eternal light. (Paul VI. Apostolic letter, Credo of the People of God, no. 9 June 30, 1968)

This Holy Trinity according to common essence undivided, and according to personal properties distinct, granted the doctrine of salvation to the human race, first through Moses and the holy prophets and his other servants according to the most methodical disposition of the time. And finally the only begotten Son of God, Jesus Christ, incarnate by the whole Trinity in common, conceived of Mary ever Virgin with the Holy Spirit cooperating, made true man, formed of a rational soul and human flesh, one Person in two natures, clearly pointed out the way of life. (Denzinger-Hünermann 800-801. Lateran Council IV 1215: Ecumenical XII, The Catholic Faith, November 11-30, 1215)

He has said: Teach ye all nations, baptising them in the name of the Father, and of the Son, and of the Holy Ghost; and again, there are three who give testimony in heaven, the Father, the Word, and the Holy Ghost; and these three are one. (Catechism of Trent, no. 1013, The Father, pg. 39)

The divine essence is not only really the same as one person, but it is really the same as the three persons. Whence, one person, and two, and three, can be predicated of the essence as if we were to say, ‘The essence is the Father, and the Son, and the Holy Ghost’. And because this word ‘God’ can of itself stand for the essence, as above explained (4, ad 3), hence, as it is true to say, ‘The essence is the three persons’; so likewise it is true to say, ‘God is the three persons’. (Saint Thomas Aquinas. Summa Theologica, I, q. 39, a. 6)

The mystery of the Most Holy Trinity is the central mystery of Christian faith and life. It is the mystery of God in himself. It is therefore the source of all the other mysteries of faith, the light that enlightens them. It is the most fundamental and essential teaching in the ‘hierarchy of the truths of faith’. (Catechism of the Catholic Church, no. 234)

This then is the salvation of Christians, that believing in the Trinity, that is, in the Father, and in the Son, and in the Holy Spirit, [and] baptized in this, we believe without doubt that there is only one true divinity and power, majesty and substance of the same. (Denzinger-Hünermann 177. Synod of Rome, 382)

Faith is certain. It is more certain than all human knowledge because it is founded on the very word of God who cannot lie. (Catechism of the Catholic Church, no. 157)

For who does not know, or cannot know that all faith is to be given to God who speaks, and that nothing is more suitable to reason itself than to acquiesce and firmly adhere to those truths which it has been established were revealed by God, who can neither deceive nor be deceived? (Denzinger-Hünermann 2778. Pius IX, Encyclical Faith and Reason Qui pluribus, November 9, 1846)

Since man is wholly dependent on God as his Creator and Lord, and since created reason is completely subject to uncreated truth, we are bound by faith to give full obedience of intellect and will to God who reveals [can. 1]. But the Catholic Church professes that this faith, which ‘is the beginning of human salvation’ [cf. n. 801], is a supernatural virtue by which we, with the aid and inspiration of the grace of God, believe that the things revealed by Him are true, not because the intrinsic truth of the revealed things has been perceived by the natural light of reason, but because of the authority of God Himself who reveals them, who can neither deceive nor be deceived [can. 2]. (Denzinger-Hünermann 3008. Vatican Council I, Session III, Dogmatic Constitution concerning the Catholic Faith, April 23, 1870)

We should be satisfied with the assurance and certitude which faith gives us that we have been taught these truths by God Himself, to doubt whose word is the extreme of folly and misery. (Catechism of Trent, 1013, The Father, pg. 39)

If anyone does not confess that (there is) one nature or substance of the Father and of the Son and of the Holy Spirit, and one power and one might, and that the Trinity is consubstantial, one Godhead being worshipped in three substances, or persons, let such a one be anathema. (Denzinger-Hünermann 421. Council of Constantinople II, Session III, June 2, 553)

We must believe in no one but God: the Father, the Son and the Holy Spirit. (Catechism of the Catholic Church, no. 178)

Because in that only true God, which is Trinity, it is naturally true not only that he is only one God, but also that he is Trinity, thus the same true God is Trinity in persons, and is only one in nature. (Saint Augustine of Hippo, On Faith, dedicated to Peter, Ch. 1, no. 4 – attr.)

The sacrosanct Roman Church, founded by the voice of our Lord and Savior, firmly believes, professes, and preaches one true God omnipotent, unchangeable, and eternal, Father, Son, and Holy Ghost; one in essence, three in persons; Father unborn, Son born of the Father, Holy Spirit proceeding from Father and Son. (Denzinger-Hünermann 1330. Council of Florence, Bull Cantata Domino, February 4, 1442)

This Trinity, distinct in Persons, [is] one indivisible and undifferentiated substance, [a united substance, indivisible and undifferentiated in its] strength, power, and majesty; [we believe that] apart from it [this] there is no nature that is divine, whether of an angel or of a spirit or of any other power, which may be believed to be God. (Denzinger-Hünermann 188. Synod of Toledo, Symbolum Toletanum I, September of 400)

Beware, Venerable Brethren, of that growing abuse, in speech as in writing, of the name of God as though it were a meaningless label, to be affixed to any creation, more or less arbitrary, of human speculation. Use your influence on the Faithful, that they refuse to yield to this aberration. Our God is the Personal God, supernatural, omnipotent, infinitely perfect, one in the Trinity of Persons, tri-personal in the unity of divine essence, the Creator of all existence. Lord, King and ultimate Consummator of the history of the world, who will not, and cannot, tolerate a rival God by His side. (Pius XI. Encyclical Mit brennender sorge, no. 9, March 14, 1937)

He [Mohammed] says that there is one God, Creator of all things created, Who has neither been begotten nor has begotten. (Saint John Damascene. Concering Heresies, no. 101: PG 94, 766)

The Christian faith principally consists in acknowledging the holy Trinity, and it specially glories in the cross of our Lord Jesus Christ. […] The following are the things you say the Muslims attack and ridicule: They ridicule the fact that we say Christ is the Son of God, when God has no wife (Qur’ân 6:110; 72:3); and they think we are insane for professing three persons in God, even though we do not mean by this three gods. (Saint Thomas Aquinas. Compendium on Reasons for the Faith against Muslim Objections, Ch. 1)

There is also the superstition of the Ishmaelites which until this day prevails and keeps people in error, being a forerunner of the Antichrist. They are descended from Ishmael, was born to Abraham of Agar, and for this reason they are called both Agarenes and Ishmaelites. […] And so down to the time of Heraclius they were very great idolaters. From that time to the present a false prophet named Mohammed has appeared in their midst. This man, after having chanced upon the Old and New Testaments and likewise, it seems, having conversed with an Arian monk, devised his own heresy. (Saint John Damascene. Concerning Heresies, 101: PG 94, 765-766)

For since they hold it for certain that men destitute of all religious sense are very rarely to be found, they seem to have founded on that belief a hope that the nations, although they differ among themselves in certain religious matters, will without much difficulty come to agree as brethren in professing certain doctrines, which form as it were a common basis of the spiritual life. For which reason conventions, meetings and addresses are frequently arranged by these persons, at which a large number of listeners are present, and at which all without distinction are invited to join in the discussion, both infidels of every kind, and Christians, even those who have unhappily fallen away from Christ or who with obstinacy and pertinacity deny His divine nature and mission. Certainly such attempts can nowise be approved by Catholics, founded as they are on that false opinion which considers all religions to be more or less good and praiseworthy, since they all in different ways manifest and signify that sense which is inborn in us all, and by which we are led to God and to the obedient acknowledgment of His rule. Not only are those who hold this opinion in error and deceived, but also in distorting the idea of true religion they reject it, and little by little. turn aside to naturalism and atheism, as it is called; from which it clearly follows that one who supports those who hold these theories and attempt to realize them, is altogether abandoning the divinely revealed religion. (Pius XI. Encyclical Mortalium animos, no. 2-3, January 6, 1928)

For the Modernist believer, on the contrary, it is an established and certain fact that the divine reality does really exist in itself and quite independently of the person who believes in it. If you ask on what foundation this assertion of the Believer rests, they answer: In the experience of the individual. […] Here it is well to note at once that, given this doctrine of experience united with the other doctrine of symbolism, every religion, even that of paganism, must be held to be true. What is to prevent such experiences from being met within every religion? In fact that they are to be found is asserted by not a few. And with what right will Modernists deny the truth of an experience affirmed by a follower of Islam? With what right can they claim true experiences for Catholics alone? Indeed Modernists do not deny but actually admit, some confusedly, others in the most open manner, that all religions are true. That they cannot feel otherwise is clear. For on what ground, according to their theories, could falsity be predicated of any religion whatsoever? (Pius X. Encyclical Pascendi Dominici gregis, no. 14, September 8, 1907)

In spirit and in truth [But the hour is coming, and is now here, when true worshipers will worship the Father in spirit and in truth. Jn 4:23] can be understood in a third way, as indicating the characteristics of true worship. For two things are necessary for a true worship: one is that the worship be spiritual; […] Secondly, the worship should be in truth. First, in the truth of faith, because no fervent spiritual desire is meritorious unless united to the truth of faith […] This prayer, then, requires three things: first, the fervor of love; secondly, the truth of faith. (Saint Thomas Aquinas. Commentary on the Gospel of Saint John, Ch. 4, Lecture 2, no. 611)

As to his saying, ‘You people worship’, [‘what you do not understand’ Jn 4:22] and so on, it should be pointed out that, as the Philosopher says, knowledge of complex things is different than knowledge of simple things. For something can be known about complex things in such a way that something else about them remains unknown; thus there can be false knowledge about them. For example, if someone has true knowledge of an animal as to its substance, he might be in error touching the knowledge of one of its accidents, such as whether it is black or white; or of a difference, such as whether it has wings or is four-footed. But there cannot be false knowledge of simple things: because they are either perfectly known inasmuch as their quiddity is known; or they are not known at all, if one cannot attain to a knowledge of them. Therefore, since God is absolutely simple, there cannot be false knowledge of him in the sense that something might be known about him and something remain unknown, but only in the sense that knowledge of him is not attained. Accordingly, anyone who believes that God is something that he is not, for example, a body, or something like that, does not adore God but something else, because he does not know him, but something else. (Saint Thomas Aquinas. Commentary on the Gospel of Saint John, Ch. 4, Lecture 2, no. 603)

For this reason, the distinction between theological faith and belief in the other religions, must be firmly held. […] This distinction is not always borne in mind in current theological reflection. Thus, theological faith (the acceptance of the truth revealed by the One and Triune God) is often identified with belief in other religions, which is religious experience still in search of the absolute truth and still lacking assent to God who reveals himself. This is one of the reasons why the differences between Christianity and the other religions tend to be reduced at times to the point of disappearance. (Congregation for the Doctrine of the Faith, Declaration Dominus Iesus, no. 7, August 6, 2000)

Jesus said to him, ‘I am the way and the truth and the life. No one comes to the Father except through me. If you know me, then you will also know my Father. From now on you do know him and have seen him.’ […] Whoever has seen me has seen the Father. (Jn 14:6-7.9)

Who is the liar? Whoever denies that Jesus is the Christ. Whoever denies the Father and the Son, this is the antichrist. No one who denies the Son has the Father, but whoever confesses the Son has the Father as well. (1 Jn 2:22-23)

He [the Father] has given all judgment to his Son, so that all may honor the Son just as they honor the Father. Whoever does not honor the Son does not honor the Father who sent him. (Jn 5:22-23)

Whoever hates me also hates my Father. (Jn 15:23)

At that time Jesus said in reply, ‘I give praise to you, Father, Lord of heaven and earth, for although you have hidden these things from the wise and the learned you have revealed them to the childlike. Yes, Father, such has been your gracious will. All things have been handed over to me by my Father. No one knows the Son except the Father, and no one knows the Father except the Son and anyone to whom the Son wishes to reveal him’. (Mt 11: 25-27)

‘In many and various ways God spoke of old to our fathers by the prophets, but in these last days he has spoken to us by a Son’ (Heb 1:1-2). No one knows the Father except the Son and anyone to whom the Son wishes to reveal him. In him he has said everything; there will be no other word than this one. Saint John of the Cross, among others, commented strikingly on Hebrews 1:1-2 […] Christian faith cannot accept ‘revelations’ that claim to surpass or correct the Revelation of which Christ is the fulfilment, as is the case in certain nonChristian religions and also in certain recent sects which base themselves on such ‘revelations’. (Catechism of the Catholic Church, no. 65.67)

For, in giving us, as He did, His Son, which is His Word – and He has no other – He spake to us all together, once and for all, in this single Word, and He has no occasion to speak further. […] That which God spake of old in the prophets to our fathers, in sundry ways and divers manners, He has now, at last, in these days, spoken to us once and for all in the Son. Herein the Apostle declares that God has become, as it were, dumb, and has no more to say, since that which He spake aforetime, in part to the prophets, He has now spoken altogether in Him, giving us the All, which is His Son. Wherefore he that would now enquire of God, or seek any vision or revelation, would not only be acting foolishly, but would be committing an offence against God, by setting his eyes altogether upon Christ, and seeking no new thing or aught beside. (Saint John of the Cross. Ascent of Carmel, Book 2, Ch. 22, no. 4-5)

For the Father has indignation when the Only-begotten Son is set at nought. For it is grievous to a king that merely his soldier should be dishonoured; and when one of his nobler officers or friends is dishonoured, then his anger is greatly increased: but if any should do despite to the king’s only-begotten son himself, who shall appease the father’s indignation on behalf of his only-begotten Son? If, therefore, any one wishes to show piety towards God, let him worship the Son, since otherwise the Father accepts not his service. (Saint Cyril of Jerusalem. Catechetical Lecture 10, One Lord Jesus Christ, no. 1-2)

It is not only necessary for Christians to believe in one God who is the Creator of heaven and earth and of all things; but also they must believe that God is the Father and that Christ is the true Son of God. This, as Saint Peter says, is not mere fable, but is certain and proved by the word of God on the Mount of Transfiguration. “For we have not by following artificial fables made known to you the power and presence of our Lord Jesus Christ; but we were eyewitnesses of His greatness. For He received from God the Father honor and glory, this voice coming down to Him from the excellent glory: ‘This is My beloved Son, in whom I am well pleased. Listen to Him.’ And this voice, we heard brought from heaven, when we were with Him in the holy mount” (2Pet 1:16). (Saint Thomas Aquinas. Expositio in Symbolum Apostolorum The Apostoles’ Creed, a.2)

We also know that the Son of God has come and has given us discernment to know the one who is true. And we are in the one who is true, in his Son Jesus Christ. He is the true God and eternal life. (1 Jn 5: 20)

A twofold commandment of faith: to believe in God and to believe in Jesus. In fact, the Lord said to his disciples: ‘Believe in God, believe also in me’ (Jn 14:1). They are not two separate acts but one single act of faith, full adherence to salvation wrought by God the Father through his Only-begotten Son. The New Testament puts an end to the Father’s invisibility. God has shown his face, as Jesus’ answer to the Apostle Philip confirms: ‘He who has seen me has seen the Father’ (Jn 14:9). (Benedict XVI. Regina Caeli, May 22, 2011)

Believing in God means giving up our own prejudices and accepting the actual face in which he revealed himself: Jesus of Nazareth the man. And this process also leads to recognizing him and to serving him in others. (Benedict XVI. Angelus, February 3, 2013)

For a Christian, believing in God cannot be separated from believing in the One he sent, his ‘beloved Son’, in whom the Father is ‘well pleased’; God tells us to listen to him. The Lord himself said to his disciples: ‘Believe in God, believe also in me’ (Jn 14:1). We can believe in Jesus Christ because he is himself God, the Word made flesh: ‘No one has ever seen God; the only Son, who is in the bosom of the Father, he has made him known’ (Jn 1:18). Because he ‘has seen the Father’, Jesus Christ is the only one who knows him and can reveal him (Jn 6:46; cf. Mt 11:27). (Catechism of the Catholic Church, no. 151)

Christ is the one Savior of all, the only one able to reveal God and lead to God. […] No one, therefore, can enter into communion with God except through Christ, by the working of the Holy Spirit. Christ’s one, universal mediation, far from being an obstacle on the journey toward God, is the way established by God himself, a fact of which Christ is fully aware. (John Paul II. Encyclical Redemptoris missio, no. 5, December 7, 1990)

In effect, the faith that the holy patriarchs and prophets received by divine inspiration before the Incarnation of the Son of God, the faith, that the holy Apostles heard also of the same Incarnate Lord, and instructed by the magisterium of the Holy Spirit, they preached not only by words, but also they left fixed in their writings for the salutary instruction of their followers, faith that preaches that the Trinity is but one God, that is to say, Father Son and Holy Spirit. (Saint Augustine of Hippo. On Faith, dedicated to Peter, Ch. 1, no. 4 – attri.)

Now the promises were made to Abraham and to his descendant. It does not say, ‘And to descendants’, as referring to many, but as referring to one, ‘And to your descendant’, who is Christ. […] And if you belong to Christ, then you are Abraham’s descendant, heirs according to the promise. (Gal 3:16, 29)

He [Mohammed] says that the Christ is the Word of God and His Spirit, but a creature and a servant, and that He was begotten, without seed, of Mary the sister of Moses and Aaron. For, he says, the Word and God and the Spirit entered into Mary and she brought forth Jesus, who was a prophet and servant of God. (Saint John Damascene. Concerning Heresies, 101: PG 94, 766)

And he says this, that when the Christ had ascended into heaven God asked Him: “O Jesus, didst thou say: ‘I am the Son of God and God’?”And Jesus, he says, answered: ‘Be merciful to me, Lord. Thou knowest that I did not say this and that I did not scorn to be thy servant. But sinful men have written that I made this statement, and they have lied about me and have fallen into error.’ (Saint John Damascene. Concerning Heresies, 101. PG 94, 766)

First of all we must observe that Muslims are silly in ridiculing us for holding that Christ is the Son of the living God, as if God had a wife. Since they are carnal, they can think only of what is flesh and blood. For any wise man can observe that the mode of generation is not the same for everything, but generation applies to each thing according to the special manner of its nature. […] So generation should be understood of God as it applies to an intellectual nature. (Saint Thomas Aquinas. Compendium on Reasons for the Faith against Muslim Objections, Ch. 3)

Moreover, they call us heretics, or Associators, because, they say, we introduce an associate with God by declaring Christ to the Son of God and God. We say to them in rejoinder: […] ‘As long as you say that Christ is the Word of God and Spirit, why do you accuse us of being heretics? For the word, and the spirit, is inseparable from that in which it naturally has existence. Therefore, if the Word of God is in God, then it is obvious that He is God. If, however, He is outside of God, then, according to you, God is without word and without spirit. Consequently, by avoiding the introduction of an associate with God you have mutilated Him. It would be far better for you to say that He has an associate than to mutilate Him, as if you were dealing with a stone or a piece of wood or some other inanimate object. Thus, you speak untruly when you call us heretics; we retort by calling you Mutilators of God.’ (Saint John Damascene. Concerning Heresies, 101. PG 94, 767)

For the universal good stands higher than any particular good, just as ‘the good of the people is better than the good of an individual’, since the goodness and perfection of the whole stand higher than the goodness and perfection of the part. But the divine goodness is compared to all others as the universal good to a particular good, being, as we have shown, the good of every good. God is, therefore, the highest good. (Saint Thomas Aquinas. Summa Contra Gentiles, Book 1, Ch. 41, no. 2)

For God does things because He wills so to do; yet the power to do them does not come from His will, but from His nature. (Saint Thomas Aquinas. Summa Theologica I, q. 25, a. 5, ad 1)

For to the will the cause of its willing is the end. But the end of the divine will is its goodness. Hence, it is the cause of God’s willing, just as it is also His act of will. […] Through the foregoing is set aside the error of certain persons who said that all things proceed from God according to His simple will, which means that we are not to give an explanation of anything except that God wills it. This view is likewise opposed to Sacred Scripture, which proclaims that God made all things according to the order of His wisdom, as is said in the Psalm (103:24): ‘You made all things in wisdom’. And in Sirach (1:10) it is said that God ‘poured’ His wisdom ‘out upon all His works’. (Saint Thomas Aquinas. Summa contra Gentiles, Book 1, Ch. 87, nos. 2.5-6)

For the virtue of a being is that by which he operates well. Now every operation of God is an operation of virtue, since His virtue is His essence, as was shown above. Therefore, God cannot will evil.
Again, the will never aims at evil without some error existing in the reason, at least with respect to a particular object of choice. For, since the object of the will is the apprehended good, the will cannot aim at evil unless in some way it is proposed to it as a good; and this cannot take place without error. But in the divine knowledge there cannot be error, as was shown above. God’s will cannot, therefore, tend towards evil.
Moreover, God is the highest good, as has been shown. But the highest good cannot bear any mingling with evil, as neither can the highest hot thing bear any mingling with the cold. The divine will, therefore, cannot be turned to evil. Furthermore, since the good has the nature of an end, evil cannot enter the will except by turning away from the end. But the divine will cannot be turned from the end, since it can will nothing except by willing itself. Therefore, it cannot will evil. (Saint Thomas Aquinas. Summa contra Gentiles, Book 1, Ch. 95, no. 2-5)

Again, as was shown above, in willing His own being, which is His own goodness, God wills all other things in so far as they bear His likeness. But in so far as a thing is opposed to the nature of being as such, there cannot be preserved in it the likeness of the first being, namely, the divine being, which is the source of being. Hence, God cannot will something that is opposed to the nature of being as such. But just as it is opposed to the nature of man as man to be irrational, so it is opposed to the nature of being as such that something be at once being and nonbeing. God, therefore, cannot will. But this is included in everything that is of itself impossible, which has an opposition with itself as implying a contradiction. The will of God, therefore, cannot be of that which is of itself impossible. (Saint Thomas Aquinas. Summa contra Gentiles, Book 1, Ch. 84, no. 3)

Is the conviction that acting unreasonably contradicts God’s nature merely a Greek idea, or is it always and intrinsically true? I believe that here we can see the profound harmony between what is Greek in the best sense of the word and the biblical understanding of faith in God. Modifying the first verse of the Book of Genesis, the first verse of the whole Bible, John began the prologue of his Gospel with the words: ‘In the beginning was the λόγος’. This is the very word used by the emperor: God acts, σὺν λόγω, with logos. Logos means both reason and word – a reason which is creative and capable of self-communication, precisely as reason. John thus spoke the final word on the biblical concept of God, and in this word all the often toilsome and tortuous threads of biblical faith find their culmination and synthesis. In the beginning was the logos, and the logos is God, says the Evangelist. The encounter between the Biblical message and Greek thought did not happen by chance. […] A profound encounter of faith and reason is taking place here, an encounter between genuine enlightenment and religion. From the very heart of Christian faith and, at the same time, the heart of Greek thought now joined to faith, Manuel II was able to say: Not to act ‘with logos’ is contrary to God’s nature. (Benedict XVI. Address at the University of Regensburg, September 12, 2006)

Accordingly, to say, if God is almighty, let Him make what has been done to be undone, is in fact to say, if God is almighty, let Him make a thing to be in the same sense both true and false. […] This truth cannot be contradicted by God, in whom abides the supreme and unchangeable truth, and whose illumination is the source of all the truth to be found in any mind or understanding. (Saint Augustine of Hippo. Reply to Faustus the Maniquean, Book 26, no. 5)

Besides, all wisdom and goodness in man are derived from the wisdom and goodness of God, as a certain likeness of Him. But it is incompatible with human wisdom and goodness to cause anyone to sin; much more, then, is it incompatible with these divine qualities. […] Hence, it is said in Sirach (Sir 15:12): ‘Say not: He caused me to err. For He has no need of wicked men’. And later: ‘He commanded no man to act wickedly, and He has given no man license to sin’ (Sir 15:21). And in James (Jas 1:13) it is said: ‘Let no man, when he is tempted, say that he is tempted by God: for God is not a tempter of evils’. (Saint Thomas Aquinas. Summa contra gentiles, Book 3, Ch. 162, nos. 4.6)

God’s Providence does not destroy the nature and order of things, but preserves them. So God’s wisdom was most evident in his preserving the order of justice and of nature. (Saint Thomas Aquinas. Compendium on Reasons for the Faith against Muslim Objections, Ch. 7)

It is the one and the same God who establishes and guarantees the intelligibility and reasonableness of the natural order of things […] and who reveals himself as the Father of our Lord Jesus Christ. This unity of truth, natural and revealed, is embodied in a living and personal way in Christ, as the Apostle reminds us: ‘Truth is in Jesus’ (cf. Eph 4:21; Col 1:15-20). He is the eternal Word in whom all things were created, and he is the incarnate Word who in his entire person reveals the Father (cf. Jn 1:14, 18). (John Paul II. Encyclical Fides et ratio, no. 34, September 14, 1998)

I was reminded of all this recently, when I read the edition by Professor Theodore Khoury (Münster) of part of the dialogue carried on – perhaps in 1391 in the winter barracks near Ankara – by the erudite Byzantine emperor Manuel II Paleologus and an educated Persian on the subject of Christianity and Islam, and the truth of both. […] The decisive statement in this argument against violent conversion is this: not to act in accordance with reason is contrary to God’s nature. The editor, Theodore Khoury, observes: For the emperor, as a Byzantine shaped by Greek philosophy, this statement is self-evident. But for Muslim teaching, God is absolutely transcendent. His will is not bound up with any of our categories, even that of rationality. (Benedict XVI. Address at the University of Regensburg, September 12, 2006)

There arose a voluntarism which […] led to the claim that we can only know God’s voluntas ordinata. Beyond this is the realm of God’s freedom, in virtue of which he could have done the opposite of everything he has actually done. This gives rise to positions which clearly approach those of Ibn Hazm and might even lead to the image of a capricious God, who is not even bound to truth and goodness. God’s transcendence and otherness are so exalted that our reason, our sense of the true and good, are no longer an authentic mirror of God, whose deepest possibilities remain eternally unattainable and hidden behind his actual decisions. As opposed to this, the faith of the Church has always insisted that between God and us, between his eternal Creator Spirit and our created reason there exists a real analogy, in which – as the Fourth Lateran Council in 1215 stated – unlikeness remains infinitely greater than likeness, yet not to the point of abolishing analogy and its language. God does not become more divine when we push him away from us in a sheer, impenetrable voluntarism. (Benedict XVI. Address at the University of Regensburg, September 12, 2006)

In the seventh conversation (διάλεξις – controversy) edited by Professor Khoury, the emperor touches on the theme of the holy war. The emperor must have known that surah 2, 256 reads: ‘There is no compulsion in religion’. According to some of the experts, this is probably one of the suras of the early period, when Mohammed was still powerless and under threat. But naturally the emperor also knew the instructions, developed later and recorded in the Qur’an, concerning holy war. Without descending to details, such as the difference in treatment accorded to those who have the ‘Book’ and the ‘infidels’, he addresses his interlocutor with a startling brusqueness, a brusqueness that we find unacceptable, on the central question about the relationship between religion and violence in general, saying: “Show me just what Mohammed brought that was new, and there you will find things only evil and inhuman, such as his command to spread by the sword the faith he preached.” (Benedict XVI. Address at the University of Regensburg, September 12, 2006)

Here Khoury quotes a work of the noted French Islamist R. Arnaldez, who points out that Ibn Hazm went so far as to state that God [Allah] is not bound even by his own word, and that nothing would oblige him to reveal the truth to us. Were it God’s will, we would even have to practise idolatry. (Benedict XVI. Address at the University of Regensburg, September 12, 2006)

Mohammed once had a friend named Zeid. This man had a beautiful wife with whom Mohammed fell in love. Once, when they were sitting together, Mohammed said: ‘Oh, by the way, God has commanded me to take your wife.’ The other answered: ‘You are an apostle. Do as God has told you and take my wife.’ Rather – to tell the story over from the beginning – he said to him: ‘God has given me the command that you put away your wife.’ And he put her away. Then several days later: ‘Now,’ he said, ‘God has commanded me to take her.’ Then, after he had taken her and committed adultery with her, he made this law: ‘Let him who will put away his wife. And if, after having put her away, he should return to her, let another marry her. For it is not lawful to take her unless she have been married by another. Furthermore, if a brother puts away his wife, let his own brother marry her, should he so wish.’ (Saint John Damascene. Concerning Heresies, 101. PG 94, 767)

Concerning merit, which depends on free will, you assert that the Muslims and other nations hold that God’s fore-knowledge or decree imposes necessity on human actions; thus they say that man cannot die or even sin unless God decrees this, and that every person has his destiny written on his forehead. (Saint Thomas Aquinas. Compendium on Reasons for the Faith against Muslim Objections, Ch. 1)

The second section of the Creed tells us more. This creative Reason is Goodness, it is Love. It has a face. God does not leave us groping in the dark. He has shown himself to us as a man. In his greatness he has let himself become small. ‘Whoever has seen me has seen the Father’, Jesus says (Jn 14:9). God has taken on a human face. He has loved us even to the point of letting himself be nailed to the Cross for our sake, in order to bring the sufferings of mankind to the very heart of God. Today, when we have learned to recognize the pathologies and the life-threatening diseases associated with religion and reason, and the ways that God’s image can be destroyed by hatred and fanaticism, it is important to state clearly the God in whom we believe, and to proclaim confidently that this God has a human face. (Benedict XVI. Homily, Islinger Feld, Regensburg, September 12, 2006)

The truly divine God is the God who has revealed himself as logos and, as logos, has acted and continues to act lovingly on our behalf. Certainly, love, as Saint Paul says, ‘transcends’ knowledge and is thereby capable of perceiving more than thought alone (cf. Eph 3:19); nonetheless it continues to be love of the God who is Logos. Consequently, Christian worship is, again to quote Paul – ‘λογικη λατρεία’, worship in harmony with the eternal Word and with our reason (cf. Rom 12:1). (Benedict XVI. Meeting with the representatives of Science, University of Regensburg, September 12, 2006)

The will of God is entirely unchangeable. […] whereas the will would be changed, if one should begin to will what before he had not willed; or cease to will what he had willed before. This cannot happen, unless we presuppose change either in the knowledge or in the disposition of the substance of the willer. For since the will regards good, a man may in two ways begin to will a thing. In one way when that thing begins to be good for him, and this does not take place without a change in him. Thus when the cold weather begins, it becomes good to sit by the fire; though it was not so before. In another way when he knows for the first time that a thing is good for him, though he did not know it before; hence we take counsel in order to know what is good for us. Now it has already been shown that both the substance of God and His knowledge are entirely unchangeable (9, 1; 14, 15). Therefore His will must be entirely unchangeable. (Saint Thomas Aquinas. Summa Theologica, I, q. 19, a. 7)

This saying is trustworthy: If we have died with him we shall also live with him; if we persevere we shall also reign with him. But if we deny him he will deny us. If we are unfaithful he remains faithful, for he cannot deny himself. (2Tim 2: 11-13)

God is not man that he should speak falsely, nor human, that he should change his mind. Is he one to speak and not act, to decree and not fulfill? (Num 23:19)

As has been related, this Mohammed wrote many ridiculous books, to each one of which he set a title. For example, there is the book ‘On Woman’, in which he plainly makes legal provision for taking four wives and, if it be possible, a thousand concubines -as many as one can maintain, besides the four wives. He also made it legal to put away whichever wife one might wish, and, should one so wish, to take to oneself another in the same way. (Saint John Damascene. Concerning Heresies, 101: PG 94, 770)

The Pharisees approached and asked, ‘Is it lawful for a husband to divorce his wife?’ They were testing him. He said to them in reply, ‘What did Moses command you?’ They replied, ‘Moses permitted him to write a bill of divorce and dismiss her.’ But Jesus told them, ‘Because of the hardness of your hearts he wrote you this commandment. But from the beginning of creation, ‘God made them male and female. For this reason a man shall leave his father and mother (and be joined to his wife), and the two shall become one flesh.’ So they are no longer two but one flesh. Therefore what God has joined together, no human being must separate.” In the house the disciples again questioned him about this. He said to them, “Whoever divorces his wife and marries another commits adultery against her; and if she divorces her husband and marries another, she commits adultery.” (Mk 10: 2-12)

Furthermore, the highest perfection of man cannot lie in a union with things inferior to himself, but, rather, in a union with some reality of a higher character, for the end is better than that which is for the sake of the end. Now, the aforementioned pleasures consist in this fact: that man is, through his senses, united with some things that are his inferiors, that is, with certain sensible objects. So, felicity is not to be located in pleasures of this sort. […] Furthermore, the ultimate end of everything is God, as is clear from what has been indicated earlier. So, we should consider the ultimate end of man to be that whereby be most closely approaches God. But, through the aforesaid pleasures, man is kept away from a close approach to God, for this approach is effected through contemplation, and the aforementioned pleasures are the chief impediment to contemplation, since they plunge man very deep into sensible things, consequently distracting him from intelligible objects. Therefore, human felicity must not be located in bodily pleasures. […] Refuted, too, are the fables of the Jews and the Saracens, who identified the rewards for just men with these pleasures, for felicity is the reward for virtue. (Saint Thomas Aquinas. Suma contra gentiles, Book 3, Ch. 27, nos. 6.10.13)

Some Sadducees, who say there is no resurrection, came to him and put this question to him, saying, “Teacher, Moses wrote for us, ‘If someone’s brother dies, leaving a wife but no child, his brother must take the wife and raise up descendants for his brother.’ Now there were seven brothers. The first married a woman and died, leaving no descendants. So the second married her and died, leaving no descendants, and the third likewise. And the seven left no descendants. Last of all the woman also died. At the resurrection (when they arise) whose wife will she be? For all seven had been married to her.” Jesus said to them, ‘Are you not misled because you do not know the scriptures or the power of God? When they rise from the dead, they neither marry nor are given in marriage, but they are like the angels in heaven’. (Mk 12:18-25)

This is the will of God, your holiness: that you refrain from immorality, that each of you know how to acquire a wife for himself in holiness and honor, not in lustful passion as do the Gentiles who do not know God; not to take advantage of or exploit a brother in this matter, for the Lord is an avenger in all these things, as we told you before and solemnly affirmed. For God did not call us to impurity but to holiness. Therefore, whoever disregards this, disregards not a human being but God, who (also) gives his holy Spirit to you. (1Thess 4:3-8)

Now the works of the flesh are obvious: immorality, impurity, licentiousness, idolatry, sorcery, hatreds, rivalry, jealousy, outbursts of fury, acts of selfishness, dissensions, factions, occasions of envy, 15 drinking bouts, orgies, and the like. I warn you, as I warned you before, that those who do such things will not inherit the kingdom of God. (Gal 5: 19-21)

God is in no way, directly or indirectly, the cause of moral evil. (Cf. Saint Augustine, De libero arbitrio I, 1, 2; Saint Thomas Aquinas, STh I-II, 79, 1) He permits it, however, because he respects the freedom of his creatures and, mysteriously, knows how to derive good from it. (Catechism of the Catholic Church, no. 311)

For the Almighty God, who, as even the heathen acknowledge, has supreme power over all things, being Himself supremely good, would never permit the existence of anything evil among His works, if He were not so omnipotent and good that He can bring good even out of evil. (Saint Augustine of Hippo. The Handbook on Faith, Hope and Love, Ch. 11)

It is impossible that any evil, as such, should be sought for by the appetite, either natural, or animal, or by the intellectual appetite which is the will. Nevertheless evil may be sought accidentally, so far as it accompanies a good […] Now the evil that accompanies one good, is the privation of another good. Never therefore would evil be sought after, not even accidentally, unless the good that accompanies the evil were more desired than the good of which the evil is the privation. Now God wills no good more than He wills His own goodness; yet He wills one good more than another. Hence He in no way wills the evil of sin, which is the privation of right order towards the divine good. The evil of natural defect, or of punishment, He does will, by willing the good to which such evils are attached. Thus in willing justice He wills punishment; and in willing the preservation of the natural order, He wills some things to be naturally corrupted. (Saint Thomas Aquinas. Summa Theologica, I, q. 19, a. 9)

In time we can discover that God in his almighty providence can bring a good from the consequences of an evil, even a moral evil, caused by his creatures: ‘It was not you’, said Joseph to his brothers, ‘who sent me here, but God. . . You meant evil against me; but God meant it for good, to bring it about that many people should be kept alive’ (Gen 45:8; ⇒ 50:20; cf. Tob 2:12 (Vulgate)). From the greatest moral evil ever committed – the rejection and murder of God’s only Son, caused by the sins of all men – God, by his grace that ‘abounded all the more’, (Rom 5:20) brought the greatest of goods: the glorification of Christ and our redemption. But for all that, evil never becomes a good. (Catechism of the Catholic Church, no. 312)

We observe that if the health of the whole body demands the excision of a member, through its being decayed or infectious to the other members, it will be both praiseworthy and advantageous to have it cut away. Now every individual person is compared to the whole community, as part to whole. Therefore if a man be dangerous and infectious to the community, on account of some sin, it is praiseworthy and advantageous that he be killed in order to safeguard the common good, since ‘a little leaven corrupteth the whole lump’ (1Cor 5:6). (Saint Thomas Aquinas. Summa Theologica II-II, q. 64, a. 2)

However, God is said by similitude to hate some things, and this in a twofold way. In the first way, because God, in loving things and by willing the existence of their good, wills the non-existence of the contrary evil. Hence, He is said to have a hatred of evils, for we are said to hate what we will not to exist. In the words of Zechariah (8:17): ‘And let none of you imagine evil in your hearts against his friend and love not a false oath. For all these are the things that I hate, says the Lord.’ These, however, are not effects in the manner of subsisting things, to which properly love and hate refer. The second way arises from the fact that God wills some greater good that cannot be without the loss of some lesser good. And thus He is said to hate, although this is rather to love. For thus, inasmuch as He wills the good of justice or of the order of the universe, which cannot exist without the punishment or corruption of some things, God is said to hate the things whose punishment or corruption He wills. In the words of Malachi (1:3): ‘I have hated Esau’; and the Psalms (5:7): ‘You hate all workers of iniquity: You destroy all who speak a lie. The bloody and the deceitful man the Lord will abhor.’ (Saint Thomas Aquinas. Summa Contra Gentiles, Book 1, Ch. 96)

The punishment of the wicked, then, which is from God, is certainly an evil to the wicked, but it is among God’s good Works, because it is just that the wicked be punished, and everything that is just is good indeed. (Saint Augustine of Hippo. Retractions, book, 1, ch. 26 – english)

For truly, the ancient inhabitants of your holy land, whom you hated for deeds most odious — works of witchcraft and impious sacrifices, a cannibal feast of human flesh and of blood. These merciless murderers of children, and parents who took with their own hands defenseless lives, You willed to destroy by the hands of our fathers, that the land that is dearest of all to you might receive a worthy colony of God’s children. But even these, as they were men, you spared, and sent wasps as forerunners of your army that they might exterminate them by degrees. Not that you were without power to have the wicked vanquished in battle by the just, or wiped out at once by terrible beasts or by one decisive word; But condemning them bit by bit, you gave them space for repentance. You were not unaware that their race was wicked and their malice ingrained, And that their dispositions would never change; for they were a race accursed from the beginning. Neither out of fear for anyone did you grant amnesty for their sins. (Wis 12: 3-11)

The first text on which we shall reflect is in chapter 18 of the Book of Genesis. It is recounted that the evil of the inhabitants of Sodom and Gomorrah had reached the height of depravity so as to require an intervention of God, an act of justice, that would prevent the evil from destroying those cities. […] Abraham confronts God with the need to avoid a perfunctory form of justice: if the city is guilty it is right to condemn its crime and to inflict punishment, but — the great Patriarch affirms — it would be unjust to punish all the inhabitants indiscriminately. If there are innocent people in the city, they must not be treated as the guilty. God, who is a just judge, cannot act in this way, Abraham says rightly to God. […] Abraham — as we remember — gradually decreases the number of innocent people necessary for salvation: if 50 would not be enough, 45 might suffice, and so on down to 10. […] However, not even 10 just people were to be found in Sodom and Gomorrah so the cities were destroyed; a destruction paradoxically deemed necessary by the prayer of Abraham’s intercession itself. Because that very prayer revealed the saving will of God: the Lord was prepared to forgive, he wanted to forgive but the cities were locked into a totalizing and paralyzing evil, without even a few innocents from whom to start in order to turn evil into good. (Benedict XVI. General audience, May 18, 2011)

They do blaspheme who say: Allah is one of three in a Trinity: for there is no god except One Allah. If they desist not from their word (of blasphemy), verily a grievous penalty will befall the blasphemers among them. (Koran, Surah 5, no. 73)

O People of the Book! Commit no excesses in your religion: Nor say of Allah aught but the truth. Christ Jesus the son of Mary was (no more than) an apostle of Allah, and His Word, which He bestowed on Mary, and a spirit proceeding from Him: so believe in Allah and His apostles. Say not ‘Trinity’ : desist: it will be better for you: for Allah is one Allah. Glory be to Him: (far exalted is He) above having a son. To Him belong all things in the heavens and on earth. And enough is Allah as a Disposer of affairs. (Koran, Surah 4, no. 171)

It is not befitting to (the majesty of) Allah that He should beget a son. Glory be to Him! when He determines a matter, He only says to it, ‘Be’, and it is. […] They say: ((Allah)) Most Gracious has begotten a son!’ Indeed ye have put forth a thing most monstrous! At it the skies are ready to burst, the earth to split asunder, and the mountains to fall down in utter ruin, That they should invoke a son for ((Allah)) Most Gracious. For it is not consonant with the majesty of ((Allah)) Most Gracious that He should beget a son. (Koran, Surah 19, nos. 35.88-89)

They do blaspheme who say: ‘(Allah) is Christ the son of Mary’. But said Christ: ‘O Children of Israel! worship Allah, my Lord and your Lord’. Whoever joins other gods with Allah,- Allah will forbid him the garden, and the Fire will be his abode. There will for the wrong-doers be no one to help. (Koran, Surah 5, no. 72)

And behold! Allah will say: ‘O Jesus the son of Mary! Didst thou say unto men, worship me and my mother as gods in derogation of Allah?’ He will say: ‘Glory to Thee! Never could I say what I had no right (to say). Had I said such a thing, thou wouldst indeed have known it. Thou knowest what is in my heart, though I know not what is in Thine. For Thou knowest in full all that is hidden. Never said I to them aught except what Thou didst command me to say, to wit, ‘worship Allah, my Lord and your Lord’; and I was a witness over them whilst I dwelt amongst them; when Thou didst take me up Thou wast the Watcher over them, and Thou art a witness to all things. (Koran, Surah 5, no. 116-117)

The similitude of Jesus before Allah is as that of Adam; He created him from dust, then said to him: ‘Be’. And he was. The Truth (comes) from Allah alone; so be not of those who doubt. (Koran, Surah 3, no. 59-60)

Say: ‘We believe in Allah, and in what has been revealed to us and what was revealed to Abraham, Isma’il, Isaac, Jacob, and the Tribes, and in (the Books) given to Moses, Jesus, and the prophets, from their Lord: We make no distinction between one and another among them, and to Allah do we bow our will (in Islam)’. (Koran, Surah 3, no. 84)

Christ the son of Mary was no more than an apostle; many were the apostles that passed away before him. (Koran, Surah 5, no. 75)

If there were, in the heavens and the earth, other gods besides Allah, there would have been confusion in both! but glory to Allah, the Lord of the Throne: (High is He) above what they attribute to Him! He cannot be questioned for His acts, but they will be questioned (for theirs). (Koran, Surah 21. No.22-23)

None of Our revelations do We abrogate or cause to be forgotten, but We substitute something better or similar: Knowest thou not that Allah Hath power over all things? (Koran, Surah 2, no. 106)

Why should ye be divided into two parties about the Hypocrites? Allah hath upset them for their (evil) deeds. Would ye guide those whom Allah hath thrown out of the Way? For those whom Allah hath thrown out of the Way, never shalt thou find the Way. (Koran, Surah 4, no. 88)

And kill them wherever you overtake them, and expel them from where they had expelled you. Oppression is more serious than murder. (Koran, Surah 2, no. 191)

And fight them on until there is no more Tumult or oppression, and there prevail justice and faith in Allah. (Koran, Surah 2, no. 193)

Those who reject our Signs, We shall soon cast into the Fire: as often as their skins are roasted through, We shall change them for fresh skins, that they may taste the penalty: for Allah is Exalted in Power, Wise. (Koran, Surah 4, no. 56)

The punishment of those who wage war against Allah and His Messenger, and strive with might and main for mischief through the land is: execution, or crucifixion, or the cutting off of hands and feet from opposite sides, or exile from the land: that is their disgrace in this world, and a heavy punishment is theirs in the Hereafter. (Koran, Surah 5, no. 33)

Your Lord inspired the angels: ‘I am with you, so support those who believe. I will cast terror into the hearts of those who disbelieve. So strike above the necks, and strike off every fingertip of theirs’. (Koran, Surah 8, no. 12)

Those who believe, and adopt exile, and fight for the Faith, in the cause of Allah as well as those who give (them) asylum and aid,- these are (all) in very truth the Believers: for them is the forgiveness of sins and a provision most generous. (Koran, Surah 8, no. 74)

But when the forbidden months are past, then fight and slay the Pagans wherever ye find them, an seize them, beleaguer them, and lie in wait for them in every stratagem (of war); but if they repent [and accept Islam], and establish regular prayers and practise regular charity, then open the way for them: for Allah is Oft-forgiving, Most Merciful. (Koran, Surah 9, no. 5)

O ye who believe! Fight the unbelievers who gird you about, and let them find firmness in you: and know that Allah is with those who fear Him. (Koran, Surah 9, no. 123)

When ye meet the Unbelievers (in fight), smite at their necks; At length, when ye have thoroughly subdued them, bind a bond firmly (on them): thereafter (is the time for) either generosity or ransom […] Soon will he [Allah] guide them and improve their condition, And admit them to the Garden which He has announced for them. (Koran, Surah 47, nos. 4.5-6)