Already in the apostolic era, the Council of Jerusalem had to unify the different perspectives of Christians of a Jewish background and those proceeding from paganism. That event continues to be a luminous testimony of how the truth should be served without compromise. (John Paul II. Angelus, no. 1, June 30, 1996)

Love for the truth is the deepest dimension of any authentic quest for full communion between Christians. […] Full communion of course will have to come about through the acceptance of the whole truth into which the Holy Spirit guides Christ’s disciples. Hence all forms of reductionism or facile ‘agreement’ must be absolutely avoided. (John Paul II. Encyclical Ut unum sint, no. 36, May 25, 1995)

Here it is not a question of altering the deposit of faith, changing the meaning of dogmas, eliminating essential words from them, accommodating truth to the preferences of a particular age, or suppressing certain articles of the Creed under the false pretext that they are no longer understood today. The unity willed by God can be attained only by the adherence of all to the content of revealed faith in its entirety. In matters of faith, compromise is in contradiction with God who is Truth. In the Body of Christ, ‘the way, and the truth, and the life’ (Jn 14:6), who could consider legitimate a reconciliation brought about at the expense of the truth? […]A ‘being together’ which betrayed the truth would thus be opposed both to the nature of God who offers his communion and to the need for truth found in the depths of every human heart. (John Paul II. Encyclical Ut unum sint, no. 18, May 25, 1995)

But far more advanced and far more pernicious are their [the modernists] teachings on doctrinal and dogmatic authority. This is their conception of the magisterium of the Church: No religious society, they say, can be a real unit unless the religious conscience of its members be one, and one also the formula which they adopt. But this double unity requires a kind of common mind whose office is to find and determine the formula that corresponds best with the common conscience, and it must have moreover an authority sufficient to enable it to impose on the community the formula which has been decided upon. From the combination and, as it were fusion of these two elements, the common mind which draws up the formula and the authority which imposes it, arises, according to the Modernists, the notion of the ecclesiastical magisterium. (Pius X. Encyclical Pascendi Dominici gregis, no. 24, September 8, 1907)

It is apparent, however, that some today, as in apostolic times, desirous of novelty, and fearing to be considered ignorant of recent scientific findings, […] try to withdraw themselves from the sacred Teaching Authority and are accordingly in danger of gradually departing from revealed truth and of drawing others along with them into error. Another danger is perceived which is all the more serious because it is more concealed beneath the mask of virtue. There are many who, deploring disagreement among men and intellectual confusion, through an imprudent zeal for souls, are urged by a great and ardent desire to do away with the barrier that divides good and honest men; these advocate an ‘eirenism’ according to which, by setting aside the questions which divide men, they aim not only at joining forces to repel the attacks of atheism, but also at reconciling things opposed to one another in the field of dogma. […] But some through enthusiasm for an imprudent ‘eirenism’ seem to consider as an obstacle to the restoration of fraternal union, things founded on the laws and principles given by Christ and likewise on institutions founded by Him, or which are the defense and support of the integrity of the faith, and the removal of which would bring about the union of all, but only to their destruction. (Pius XII. Encyclical Humani generis, no. 10.11.12, August 12, 1950)

Even on the plea of promoting unity it is not allowed to dissemble one single dogma; for, as the Patriarch of Alexandria [St Cyril] warns us, ‘although the desire of peace is a noble and excellent thing, yet we must not for its sake neglect the virtue of loyalty in Christ’ (Ep. 61). […] The only successful method will be that which bases harmony and agreement among Christ’s faithful ones upon all the truths, and the whole of the truths, which God has revealed. (Pius XII. Encyclical Orientalis Ecclesiae, no.1, April 9, 1944)

Catholic doctrine tells us that the primary duty of charity does not lie in the toleration of false ideas, however sincere they may be, nor in the theoretical or practical indifference towards the errors and vices in which we see our brethren plunged […] If, as We desire with all Our heart, the highest possible peak of well being for society and its members is to be attained through fraternity or, as it is also called, universal solidarity, all minds must be united in the knowledge of Truth, all wills united in morality, and all hearts in the love of God and His Son Jesus Christ. (Pius X. Encyclical Notre charge apostolique, August 23, 1910)

The desire to come together as brothers must not lead to a watering down or whittling away of truth. Our dialogue must not weaken our attachment to our faith. […] (irenism and syncretism) is ultimately nothing more than skepticism about the power and content of the Word of God which we desire to preach. (Paul VI. Encyclical Ecclesiam suam, no. 88, August 6, 1964)

Every evangelizer is expected to have a reverence for truth, especially since the truth that he studies and communicates is none other than revealed truth and hence, more than any other, a sharing in the first truth which is God Himself. The preacher of the Gospel will therefore be a person who even at the price of personal renunciation and suffering always seeks the truth that he must transmit to others. He never betrays or hides truth out of a desire to please men, in order to astonish or to shock, nor for the sake of originality or a desire to make an impression. He does not refuse truth. […]The God of truth expects us to be the vigilant defenders and devoted preachers of truth. (Paul VI. Apostolic exhortation Evangelii nuntiandi, no. 78, December 8, 1975)

Shall We suffer, what would indeed be iniquitous, the truth, and a truth divinely revealed, to be made a subject for compromise? For here there is question of defending revealed truth. […] If our Redeemer plainly said that His Gospel was to continue not only during the times of the Apostles, but also till future ages, is it possible that the object of faith should in the process of time become so obscure and uncertain, that it would be necessary to-day to tolerate opinions which are even incompatible one with another? If this were true, we should have to confess that the coming of the Holy Ghost on the Apostles, and the perpetual indwelling of the same Spirit in the Church, and the very preaching of Jesus Christ, have several centuries ago, lost all their efficacy and use, to affirm which would be blasphemy. (Pius XI. Encyclical Mortalium Animos, January 6, 1928)

Therefore the whole and entire Catholic doctrine is to be presented and explained: by no means is it permitted to pass over in silence or to veil in ambiguous terms the Catholic truth regarding the nature and way of justification, […] because outside the truth no true union can ever be attained. (Congregation for the Doctrine of the Faith, Instruction Ecclesia Catholica, December 20, 1949)

Unity in truth: this is the mission confided by Christ to his Church, for which it actively strives, invoking before all else from Him who is all powerful and was the first to pray to the Father, in the imminence of his Death and Resurrection, that all believers be ‘one’ (Jn 17: 21) […] It thus becomes clear the this mysterious and visible union may not be obtained without the identity of the faith, the participation in sacramental life, the consequent coherence in moral life, and continuous and fervent personal and communitarian prayer. (John Paul II. Official and solemn presentation of the Catechism of the Catholic Church, no. 8, December 7, 1992)

To be watchful for purity of doctrine, the basis in building up the Christian community, is therefore, together with the proclamation of the Gospel, the primary and irreplaceable duty of the Pastor, of the Teacher of the faith. How often Saint Paul emphasized this, convinced as he was of the seriousness of the accomplishment of this duty (cf. 1Tim 1:3-7; 18-20; 4:11, 16; 2Tim 1:4-14). Over and above unity in love, unity in truth is always urgent for us. (John Paul II. Address to the members of the Third General Conference of the Latin American Episcopate, Puebla, no. 1, January 28, 1979)

We mean a perfect and complete union, such as could not subsist in any way if nothing else was brought about but a certain kind of agreement in the Tenets of Belief and an intercourse of Fraternal love. The True Union between Christians is that which Jesus Christ, the Author of the Church, instituted and desired, and which consists in a Unity of Faith and Unity of Government. (Leo XIII. Encyclical Praeclara gratulationis, June 20, 1894)

According to Acts, the unity of believers was seen in the fact that ‘they devoted themselves to the Apostles’ teaching and fellowship, to the breaking of the bread and the prayers’ (Acts 2:42). The unity of believers was thus nourished by the teaching of the Apostles (the proclamation of God’s word), to which they responded with unanimous faith, by fraternal communion (the service of charity), by the breaking of the bread (the Eucharist and the sacraments), and by prayer, both personal and communal. It was on these four pillars that communion and witness were based within the first community of believers. (Benedict XVI. Apostolic Exhortation Ecclesia in Medio Oriente, no. 5, September 14, 2012)

It is the Holy Spirit, the principle of unity, which establishes the Church as a communion (cf. Lumen gentium, 13). He is the principle of the unity of the faithful in the teaching of the Apostles, in the breaking of the bread and in prayer (cf. ibid; Acts 2:42). […] The communion of the baptized in the teaching of the Apostles and in the breaking of the eucharistic bread is visibly manifested in the bonds of the profession of the faith in its entirety, of the celebration of all of the sacraments instituted by Christ, and of the governance of the College of Bishops united with its head, the Roman Pontiff (cf. CIC, can. 205; Lumen gentium, 13; 14; 21; 22; Unitatis redintegratio, 2; 3; 4; 15; 20; Christus Dominus, 4; Ad gentes, 22.) (Benedict XVI. Apostolic constitution Anglicanorum coetibus, November 4, 2009)

Jesus says: ‘When the Spirit of truth comes, he will guide you into all the truth’ (Jn 16:13). Here, in speaking of the Holy Spirit, Jesus explains to us what the Church is and how she should live in order to be herself, to be the place of unity and communion in Truth; […] Dear friends, we must live in accordance with the Spirit of unity and truth and this is why we should pray that the Spirit illuminate and guide us so that we may overcome our fascination with following our own truths and receive the truth of Christ, passed on in the Church. (Benedict XVI. Homily Solemnity of Pentecost, May 27, 2012)

Some who had come down from Judea were instructing the brothers, ‘Unless you are circumcised according to the Mosaic practice, you cannot be saved.’ Because there arose no little dissension and debate by Paul and Barnabas with them, it was decided that Paul, Barnabas, and some of the others should go up to Jerusalem to the apostles and presbyters about this question. (Acts 15:1-2)

A synod, which has not always been enjoined in other cases, unless something new arose in distorted minds or something ambiguous in a pronouncement so that, if there were any obscurity, the authority of sacerdotal deliberation might illumine those who were treating the ambiguous pronouncement in common, just as first the impiety of Arius and then that of Nestorius, lastly that of Dioscorus and also of Eutyches caused this to be done. (Denzinger-Hünermann 343. Simplicius, Epistle Quantum presbyterorum, January 9, 476)

Let us with all zeal avoid novel doctrines and presumptuous talkativeness, whence rather the smoke of contention and of scandal between brothers can be stirred up, than any increase of the fear of God arise. […] we believe that we must firmly hold that only which we are happy to have drawn from the maternal womb of the Church. (Denzinger-Hünermann 625, Synod of Valence III 855, Predesination, January 8, 855)

It is in the same light and power that the Church’s Magisterium continues to carry out its task of discernment, accepting and living out the admonition addressed by the Apostle Paul to Timothy: ‘I charge you in the presence of God and of Christ Jesus who is to judge the living and the dead, and by his appearing and his kingdom: preach the word, be urgent in season and out of season, convince, rebuke, and exhort, be unfailing in patience and in teaching. For the time will come when people will not endure sound teaching, but having itching ears they will accumulate for themselves teachers to suit their own likings, and will turn away from listening to the truth and wander into myths. As for you, always be steady, endure suffering, do the work of an evangelist, fulfil your ministry’ (2Tim 4:1-5; cf. Tit 1:10, 13-14). (John Paul II. Encyclical Veritatis splendor, no. 30, August 6, 1993)

The office divinely committed to Us of feeding the Lord’s flock has especially this duty assigned to it by Christ, namely, to guard with the greatest vigilance the deposit of the faith delivered to the saints, rejecting the profane novelties of words and oppositions of knowledge falsely so called. There has never been a time when this watchfulness of the supreme pastor was not necessary to the Catholic body; for, owing to the efforts of the enemy of the human race, there have never been lacking ‘men speaking perverse things’ (Acts 20:30), ‘vain talkers and seducers’ (Tit 1:10), ‘erring and driving into error’ (2Tim 3:13). (Pius X. Encyclical Pascendi Dominici gregis, no. 1, September 8, 1907)

Blind that they are, and leaders of the blind, inflated with a boastful science, they have reached that pitch of folly where they pervert the eternal concept of truth and the true nature of the religious sentiment; with that new system of theirs they are seen to be under the sway of a blind and unchecked passion for novelty, thinking not at all of finding some solid foundation of truth, but despising the holy and apostolic traditions, they embrace other vain, futile, uncertain doctrines, condemned by the Church, on which, in the height of their vanity, they think they can rest and maintain truth itself. […] But for Catholics the second Council of Nicea will always have the force of law, where it condemns those who dare, after the impious fashion of heretics, to deride the ecclesiastical traditions, to invent novelties of some kind… or endeavour by malice or craft to overthrow any one of the legitimate traditions of the Catholic Church. (Pius X. Encyclical Pascendi Dominici gregis, no. 11.42, September 8, 1907)

Nor do we merely desire that Catholics should shrink from the errors of Modernism, but also from the tendencies or what is called the spirit of Modernism. Those who are infected by that spirit develop a keen dislike for all that savours of antiquity and become eager searchers after novelties in everything: in the way in which they carry out religious functions, in the ruling of Catholic institutions, and even in private exercises of piety. Therefore it is Our will that the law of our forefathers should still be held sacred: ‘Let there be no innovation; keep to what has been handed down.’ In matters of faith that must be inviolably adhered to as the law; it may however also serve as a guide even in matters subject to change, but even in such cases the rule would hold: ‘Old things, but in a new way.’ (Benedict XV. Encyclical Ad beatissimi Apostolorum, November 1, 1914)

Among priests, especially among the less formed in doctrine and with a less austere life, circulates, in an ever more grave and preoccupying manner, a certain spirit of novelty. Novelty, in and of itself, is never a sure criteria of the truth; and can be praiseworthy only when it confirms the truth and leads to uprightness and virtue. (Pius XII. Apostolic Exhortation Menti nostrae, September 23, 1950)

… studying more closely the ideas of the Modernists, evolution is described as resulting from the conflict of two forces, one of them tending towards progress, the other towards conservation. The conserving force in the Church is tradition […] The progressive force, on the contrary, which responds to the inner needs lies in the individual consciences and ferments there […] Now it is by a species of compromise between the forces of conservation and of progress, that is to say between authority and individual consciences, that changes and advances take place. […] Thus then, Venerable Brethren, for the Modernists, both as authors and propagandists, there is to be nothing stable, nothing immutable in the Church. (Pius X. Encyclical Pascendi Dominici gregis, no. 27-28, September 8, 1907)

Even in apostolic times there were perverse men who, for personal interest and ambition, confused and corrupted among the faithful the purity of the faith with abominable errors. The Apostles opposed them with preaching, with writings and with the infallible sentences of the first Council that occurred in Jerusalem. Ever since then, until now, the spirit of darkness has not ceased in his pernicious attacks against the Church and the divine truths of which it is the indefectible guardian; and constantly arousing new heresies, it has made attempts one after the other against all of the dogmas of the Christian religion. (Catechism of Saint Pius X, no. 3126-3127)

Then after fourteen years I again went up to Jerusalem with Barnabas, taking Titus along also. I went up in accord with a revelation, and I presented to them the gospel that I preach to the Gentiles – but privately to those of repute – so that I might not be running, or have run, in vain. Moreover, not even Titus, who was with me, although he was a Greek, was compelled to be circumcised, but because of the false brothers secretly brought in, who slipped in to spy on our freedom that we have in Christ Jesus, that they might enslave us – to them we did not submit even for a moment, so that the truth of the gospel might remain intact for you. (Gal 2:1-5)

And so the apostolic preaching, which is expressed in a special way in the inspired books, was to be preserved by an unending succession of preachers until the end of time. Therefore the Apostles, handing on what they themselves had received, warn the faithful to hold fast to the traditions which they have learned either by word of mouth or by letter (2Thes 2:15), and to fight in defense of the faith handed on once and for all (Jude 1:3). (Vatican Council II, Dogmatic constitution Dei Verbum, no. 9, November 18, 1965)

Our Apostolic Mandate requires from Us that We watch over the purity of the Faith and the integrity of Catholic discipline. It requires from Us that We protect the faithful from evil and error; especially so when evil and error are presented in dynamic language which, concealing vague notions and ambiguous expressions with emotional and high-sounding words, is likely to set ablaze the hearts of men in pursuit of ideals which, whilst attractive, are nonetheless nefarious. (Pius X. Encyclical Notre charge apostolique, no. 1, August 23, 1910)

We also insisted on the grave responsibility incumbent upon us, but which we share with our Brothers in the Episcopate, of preserving unaltered the content of the Catholic faith which the Lord entrusted to the apostles. While being translated into all expressions, this content must be neither impaired nor mutilated. While being clothed with the outward forms proper to each people, and made explicit by theological expression which takes account of differing cultural, social and even racial milieu, it must remain the content of the Catholic faith just exactly as the ecclesial magisterium has received it and transmits it. (Paul VI. Apostolic Exhortation Evangelii nuntiandi, no. 65, December 8, 1975)

But it is worth while to expound the whole of that passage of the apostle more fully, ‘O Timothy, keep the deposit, avoiding profane novelties of words.’ […] Who is the Timothy of today, but either generally the Universal Church, or in particular, the whole body of the Prelacy, whom it behooves either themselves to possess or to communicate to others a complete knowledge of religion? […] What is ‘The deposit’? That which has been entrusted to you, not that which you have yourself devised: a matter not of wit, but of learning; not of private adoption, but of public tradition; a matter brought to you, not put forth by you, wherein you are bound to be not an author but a keeper, not a teacher but a disciple, not a leader but a follower. ‘Keep the deposit.’ Preserve the talent of Catholic Faith inviolate, unadulterate. That which has been entrusted to you, let it continue in your possession, let it be handed on by you. You have received gold; give gold in turn. Do not substitute one thing for another. Do not for gold impudently substitute lead or brass. Give real gold, not counterfeit. (Saint Vincent of Lerins. Commonitory, Ch. 22, no. 53)

The underlying principle of these new opinions is that, in order to more easily attract those who differ from her, the Church should shape her teachings more in accord with the spirit of the age and relax some of her ancient severity and make some concessions to new opinions. Many think that these concessions should be made not only in regard to ways of living, but even in regard to doctrines which belong to the deposit of the faith. […] It does not need many words, beloved son, to prove the falsity of these ideas if the nature and origin of the doctrine which the Church proposes are recalled to mind. […] Let it be far from anyone’s mind to suppress for any reason any doctrine that has been handed down. Such a policy would tend rather to separate Catholics from the Church than to bring in those who differ. […] History proves clearly that the Apostolic See, to which has been entrusted the mission not only of teaching but of governing the whole Church, has continued ‘in one and the same doctrine, one and the same sense, and one and the same judgment’ (Const. De fide, Chapter iv). (Leo XIII. Encyclical Testem benvolentiae nostrae, January 22, 1899)

For truth and its philosophic expression cannot change from day to day, least of all where there is question of self-evident principles of the human mind or of those propositions which are supported by the wisdom of the ages and by divine revelation. […] Let no Christian therefore, whether philosopher or theologian, embrace eagerly and lightly whatever novelty happens to be thought up from day to day, but rather let him weigh it with painstaking care and a balanced judgment, lest he lose or corrupt the truth he already has, with grave danger and damage to his faith. (Pius XII. Encyclical Humani generis, no. 30, August 12, 1950)

Furthermore, Christ instituted His Church as ‘the pillar and bulwark of truth’ (1Tm 3:15). With the Holy Spirit’s assistance, she ceaselessly preserves and transmits without error the truths of the moral order, and she authentically interprets not only the revealed positive law but ‘also . . . those principles of the moral order which have their origin in human nature itself’ (Dign Hum, 14; Pius XI, CastConn,; Pius XII, Alloc. 2/XI/1954; John XXIII, Mater et Mag; Paul VI, Hum Vit, 4) (and which concern man’s full development and sanctification. Now in fact the Church throughout her history has always considered a certain number of precepts of the natural law as having an absolute and immutable value, and in their transgression she has seen a contradiction of the teaching and spirit of the Gospel. (Congregation for the Doctrine of the Faith, Declaration Persona humana, December 29, 1975)

John Paul II

The moral prescriptions which God imparted in the Old Covenant, and which attained their perfection in the New and Eternal Covenant in the very person of the Son of God made man, must be faithfully kept and continually put into practice in the various different cultures throughout the course of history. [...] This constant ‘putting into practice’ of the commandments [...] can only confirm the permanent validity of Revelation and follow in the line of the interpretation given to it by the great Tradition of the Church’s teaching and life, as witnessed by the teaching of the Fathers, the lives of the Saints, the Church’s Liturgy and the teaching of the Magisterium. (John Paul II. Encyclical Veritatis splendor, no. 25.27, August 6, 1993)