It is part of the best agent to produce an effect which is best in its entirety; but this does not mean that He makes every part of the whole the best absolutely, but in proportion to the whole; in the case of an animal, for instance, its goodness would be taken away if every part of it had the dignity of an eye. Thus, therefore, God also made the universe to be best as a whole, according to the mode of a creature; whereas He did not make each single creature best, but one better than another. And therefore we find it said of each creature, ‘God saw the light that it was good’ (Gen 1:4); and in like manner of each one of the rest. But of all together it is said, ‘God saw all the things that He had made, and they were very good’ (Gen 1:31). (Saint Thomas Aquinas. Summa Theologica I, q. 47, a. 2, ad 2 – cf. I, q. 47, a. 1 and I, q. 47, a. 3)

It was necessary for man’s salvation that there should be a knowledge revealed by God besides philosophical science built up by human reason. Firstly, indeed, because man is directed to God, as to an end that surpasses the grasp of his reason […] But the end must first be known by men who are to direct their thoughts and actions to the end. Hence it was necessary for the salvation of man that certain truths which exceed human reason should be made known to him by divine revelation. Even as regards those truths about God which human reason could have discovered, it was necessary that man should be taught by a divine revelation; because the truth about God such as reason could discover, would only be known by a few, and that after a long time, and with the admixture of many errors. Whereas man’s whole salvation, which is in God, depends upon the knowledge of this truth. Therefore, in order that the salvation of men might be brought about more fitly and more surely, it was necessary that they should be taught divine truths by divine revelation. It was therefore necessary that besides philosophical science built up by reason, there should be a sacred science learned through revelation. (Saint Thomas Aquinas. Summa Theologica, I, q. 1, a. 1)

Truth must consequently be the ultimate end of the whole universe, and the consideration of the wise man aims principally at truth. So it is that, according to His own statement, divine Wisdom testifies that He has assumed flesh and come into the world in order to make the truth known: ‘For this was I born, and for this came I into the world, that I should give testimony to the truth’ (Jn 18:37). The Philosopher himself establishes that first philosophy is the science of truth, not of any truth, but of that truth which is the origin of all truth, namely, which belongs to the first principle whereby all things are. The truth belonging to such a principle is, clearly, the source of all truth; for things have the same disposition in truth as in being. It belongs to one and the same science, however, both to pursue one of two contraries and to oppose the other. Medicine, for example, seeks to effect health and to eliminate illness. Hence, just as it belongs to the wise man to meditate especially on the truth belonging to the first principle and to teach it to others, so it belongs to him to refute the opposing falsehood. Appropriately, therefore, is the twofold office of the wise man shown from the mouth of Wisdom in our opening words: to meditate and speak forth of the divine truth, which is truth in person (Wisdom touches on this in the words my mouth shall meditate truth), and to refute the opposing error (which Wisdom touches on in the words and my lips shall hate impiety). (Saint Thomas Aquinas. Summa contra Gentiles, Book 1, Ch. I)

For these ‘secrets of divine Wisdom’ (Job 11:6) the divine Wisdom itself, which knows all things to the full, has deigned to reveal to men. It reveals its own presence, as well as the truth of its teaching and inspiration, by fitting arguments; and in order to confirm those truths that exceed natural knowledge, it gives visible manifestation to works that surpass the ability of all nature. […] When these arguments were examined, through the efficacy of the abovementioned proof, and not the violent assault of arms or the promise of pleasure, and (what is most wonderful of all) in the midst of the tyranny of the persecutors, an innumerable throng of people, both simple and most learned, flocked to the Christian faith. In this faith there are truths preached that surpass every human intellect; the pleasures of the flesh are curbed; it is taught that the things of the world should be spurned. Now, for the minds of mortal men to assent to these things is the greatest of miracles, just as it is a manifest work of divine inspiration that, spurning visible things, men should seek only what is invisible. […]On the other hand, those who founded sects committed to erroneous doctrines proceeded in a way that is opposite to this, The point is clear in the case of Muhammad. He seduced the people by promises of carnal pleasure to which the concupiscence of the flesh goads us. His teaching also contained precepts that were in conformity with his promises, and he gave free rein to carnal pleasure. In all this, as is not unexpected, he was obeyed by carnal men. As for proofs of the truth of his doctrine, he brought forward only such as could be grasped by the natural ability of anyone with a very modest wisdom. Indeed, the truths that he taught he mingled with many fables and with doctrines of the greatest falsity. He did not bring forth any signs produced in a supernatural way, which alone fittingly gives witness to divine inspiration; for a visible action that can be only divine reveals an invisibly inspired teacher of truth. On the contrary, Muhammad said that he was sent in the power of his arms—which are signs not lacking even to robbers and tyrants. What is more, no wise men, men trained in things divine and human, believed in him from the beginning, Those who believed in him were brutal men and desert wanderers, utterly ignorant of all divine teaching, through whose numbers Muhammad forced others to become his followers by the violence of his arms. Nor do divine pronouncements on the part of preceding prophets offer him any witness. On the contrary, he perverts almost all the testimonies of the Old and New Testaments by making them into fabrications of his own, as can be seen by anyone who examines his law. It was, therefore, a shrewd decision on his part to forbid his followers to read the Old and New Testaments, lest these books convict him of falsity. It is thus clear that those who place any faith in his words believe foolishly. (Saint Thomas Aquinas. Summa contra Gentiles, Book 1, Ch. VI)

That which we hold by faith as divinely revealed, therefore, cannot be contrary to our natural knowledge. Again. In the presence of contrary arguments our intellect is chained, so that it cannot proceed to the knowledge of the truth. If, therefore, contrary knowledge were implanted in us by God, our intellect would be hindered from knowing truth by this very fact. Now, such an effect cannot come from God. And again. What is natural cannot change as long as nature does not. Now, it is impossible that contrary opinions should exist in the same knowing subject at the same time. (Saint Thomas Aquinas. Summa contra Gentiles, Book 1, Ch. VII)

A twofold difficult may attach to works of virtue with which the precepts of the Law are concerned. One is on the part of the outward works, which of themselves are, in a way, difficult and burdensome. And in this respect the Old Law is a much heavier burden than the New: since the Old Law by its numerous ceremonies prescribed many more outward acts than the New Law, which, in the teaching of Christ and the apostles, added very few precepts to those of the natural law; although afterwards some were added, through being instituted by the holy Fathers. Even in these Augustine says that moderation should be observed, lest good conduct should become a burden to the faithful. For he says in reply to the queries of Januarius (Ep. lv) that, ‘whereas God in His mercy wished religion to be a free service rendered by the public solemnization of a small number of most manifest sacraments, certain persons make it a slave’s burden; so much so that the state of the Jews who were subject to the sacraments of the Law, and not to the presumptuous devices of man, was more tolerable.’
The other difficulty attaches to works of virtue as to interior acts: for instance, that a virtuous deed be done with promptitude and pleasure. It is this difficulty that virtue solves: because to act thus is difficult for a man without virtue: but through virtue it becomes easy for him. In this respect the precepts of the New Law are more burdensome than those of the Old; because the New Law prohibits certain interior movements of the soul, which were not expressly forbidden in the Old Law in all cases, although they were forbidden in some, without, however, any punishment being attached to the prohibition. Now this is very difficult to a man without virtue: thus even the Philosopher states (Ethic. v, 9) that it is easy to do what a righteous man does; but that to do it in the same way, viz. with pleasure and promptitude, is difficult to a man who is not righteous. Accordingly we read also (1 Jn 5:3) that ‘His commandments are not heavy’: which words Augustine expounds by saying that ‘they are not heavy to the man that loveth; whereas they are a burden to him that loveth not.’ (Saint Thomas Aquinas. Summa Theologica, I-II, q. 107, a. 4)

‘Each thing appears to be that which preponderates in it,’ as the Philosopher states (Ethic. ix, 8). Now that which is preponderant in the law of the New Testament, and whereon all its efficacy is based, is the grace of the Holy Ghost, which is given through faith in Christ. Consequently the New Law is chiefly the grace itself of the Holy Ghost, which is given to those who believe in Christ. This is manifestly stated by the Apostle who says (Rom 3:27): ‘Where is . . . thy boasting? It is excluded. By what law? Of works? No, but by the law of faith’: for he calls the grace itself of faith ‘a law.’ And still more clearly it is written (Rom 8:2): ‘The law of the spirit of life, in Christ Jesus, hath delivered me from the law of sin and of death.’ Hence Augustine says (De Spir. et Lit. xxiv) that ‘as the law of deeds was written on tables of stone, so is the law of faith inscribed on the hearts of the faithful’: and elsewhere, in the same book (xxi): ‘What else are the Divine laws written by God Himself on our hearts, but the very presence of His Holy Spirit?’ Nevertheless the New Law contains certain things that dispose us to receive the grace of the Holy Ghost, and pertaining to the use of that grace: such things are of secondary importance, so to speak, in the New Law; and the faithful need to be instructed concerning them, both by word and writing, both as to what they should believe and as to what they should do. Consequently we must say that the New Law is in the first place a law that is inscribed on our hearts, but that secondarily it is a written law. (Saint Thomas Aquinas. Summa Theologica, I-II, q. 106, a. 1)

As stated above (Article [1]), there is a twofold element in the Law of the Gospel. There is the chief element, viz. the grace of the Holy Ghost bestowed inwardly. And as to this, the New Law justifies. Hence Augustine says (De Spiritu et Littera xvii): ‘There,’ i.e. in the Old Testament, ‘the Law was set forth in an outward fashion, that the ungodly might be afraid’; ‘here,’ i.e. in the New Testament, ‘it is given in an inward manner, that they may be justified.’ The other element of the Evangelical Law is secondary: namely, the teachings of faith, and those commandments which direct human affections and human actions. And as to this, the New Law does not justify. Hence the Apostle says (2Cor 3:6) ‘The letter killeth, but the spirit quickeneth’: and Augustine explains this (De Spir. et Lit. xiv, xvii) by saying that the letter denotes any writing external to man, even that of the moral precepts such as are contained in the Gospel. Wherefore the letter, even of the Gospel would kill, unless there were the inward presence of the healing grace of faith. (Saint Thomas Aquinas. Summa Theologica, I-II, q. 106, a. 2)

The New Law consists chiefly in the grace of the Holy Ghost, which is shown forth by faith that worketh through love. Now men become receivers of this grace through God’s Son made man, Whose humanity grace filled first, and thence flowed forth to us. Hence it is written (Jn 1:14): ‘The Word was made flesh,’ and afterwards: ‘full of grace and truth’; and further on: ‘Of His fulness we all have received, and grace for grace.’ Hence it is added that ‘grace and truth came by Jesus Christ.’ Consequently it was becoming that the grace flows from the incarnate Word should be given to us by means of certain external sensible objects; and that from this inward grace, whereby the flesh is subjected to the Spirit, certain external works should ensue. Accordingly external acts may have a twofold connection with grace. In the first place, as leading in some way to grace. Such are the sacramental acts which are instituted in the New Law, e.g. Baptism, the Eucharist, and the like. In the second place there are those external acts which ensue from the promptings of grace: and herein we must observe a difference. For there are some which are necessarily in keeping with, or in opposition to inward grace consisting in faith that worketh through love. Such external works are prescribed or forbidden in the New Law; thus confession of faith is prescribed, and denial of faith is forbidden; for it is written (Mt 10:32,33) ‘(Every one) that shall confess Me before men, I will also confess him before My Father . . . But he that shall deny Me before men, I will also deny him before My Father.’ On the other hand, there are works which are not necessarily opposed to, or in keeping with faith that worketh through love. Such works are not prescribed or forbidden in the New Law, by virtue of its primitive institution; but have been left by the Lawgiver, i.e. Christ, to the discretion of each individual. And so to each one it is free to decide what he should do or avoid; and to each superior, to direct his subjects in such matters as regards what they must do or avoid. (Saint Thomas Aquinas. Summa Theologica, I-II, q. 108. a. 1)

Just as it belongs to the secular authority to make legal precepts which apply the natural law to matters of common weal in temporal affairs, so it belongs to ecclesiastical superiors to prescribe by statute those things that concern the common weal of the faithful in spiritual goods. (Saint Thomas Aquinas. Summa Theologica, II-II, q. 147, a. 3)

…the salvation of souls […] must always be the supreme law in the Church… (Code of Canon Law, Can. 1752)

The precepts of the Church are set in the context of a moral life bound to and nourished by liturgical life. The obligatory character of these positive laws decreed by the pastoral authorities is meant to guarantee to the faithful the indispensable minimum in the spirit of prayer and moral effort, in the growth in love of God and neighbor. (Catechism of the Catholic Church, no. 2041)

The faithful are obliged to acknowledge and respect the specific moral precepts declared and taught by the Church in the name of God, the Creator and Lord. When the Apostle Paul sums up the fulfilment of the law in the precept of love of neighbour as oneself (cf. Rom 13:8-10), he is not weakening the commandments but reinforcing them, since he is revealing their requirements and their gravity. Love of God and of one’s neighbour cannot be separated from the observance of the commandments of the Covenant renewed in the blood of Jesus Christ and in the gift of the Spirit. It is an honour characteristic of Christians to obey God rather than men (cf. Acts 4:19; 5:29) and accept even martyrdom as a consequence, like the holy men and women of the Old and New Testaments, who are considered such because they gave their lives rather than perform this or that particular act contrary to faith or virtue.
(Note 125: Cf. Ecumenical Council of Trent, Session VI, Decree on Justification Cum Hoc Tempore, Canon 19: DS, 1569. See also: Clement XI, Constitution Unigenitus Dei Filius (September 8, 1713) against the Errors of Paschasius Quesnel, Nos. 53-56: DS, 2453-2456.) (John Paul II. Encyclical Veritatis splendor, no. 76)

Perfection demands that maturity in self-giving to which human freedom is called. Jesus points out to the young man that the commandments are the first and indispensable condition for having eternal life; on the other hand, for the young man to give up all he possesses and to follow the Lord is presented as an invitation: ‘If you wish…’. These words of Jesus reveal the particular dynamic of freedom’s growth towards maturity, and at the same time they bear witness to the fundamental relationship between freedom and divine law. Human freedom and God’s law are not in opposition; on the contrary, they appeal one to the other. The follower of Christ knows that his vocation is to freedom. ‘You were called to freedom, brethren’ (Gal 5:13), proclaims the Apostle Paul with joy and pride. But he immediately adds: ‘only do not use your freedom as an opportunity for the flesh, but through love be servants of one another’ (ibid.). The firmness with which the Apostle opposes those who believe that they are justified by the Law has nothing to do with man’s ‘liberation’ from precepts. On the contrary, the latter are at the service of the practice of love: ‘For he who loves his neighbour has fulfilled the Law. The commandments, ‘You shall not commit adultery; You shall not murder; You shall not steal; You shall not covet,’and any other commandment, are summed up in this sentence, ‘You shall love your neighbour as yourself’ (Rom 13:8-9). Saint Augustine, after speaking of the observance of the commandments as being a kind of incipient, imperfect freedom, goes on to say: ‘Why, someone will ask, is it not yet perfect? Because ‘I see in my members another law at war with the law of my reason’… In part freedom, in part slavery: not yet complete freedom, not yet pure, not yet whole, because we are not yet in eternity. In part we retain our weakness and in part we have attained freedom. All our sins were destroyed in Baptism, but does it follow that no weakness remained after iniquity was destroyed? Had none remained, we would live without sin in this life. But who would dare to say this except someone who is proud, someone unworthy of the mercy of our deliverer?… Therefore, since some weakness has remained in us, I dare to say that to the extent to which we serve God we are free, while to the extent that we follow the law of sin, we are still slaves’ (In Iohannis Evangelium Tractatus 41, 10). (John Paul II. Encyclical Veritatis splendor, no. 17)