And here it seems opportune to expound and to refute a certain false opinion, on which this whole question, as well as that complex movement by which non-Catholics seek to bring about the union of the Christian churches depends. For authors who favor this view are accustomed, times almost without number, to bring forward these words of Christ: "That they all may be one.... And there shall be one fold and one shepherd," with this signification however: that Christ Jesus merely expressed a desire and prayer, which still lacks its fulfillment. For they are of the opinion that the unity of faith and government, which is a note of the one true Church of Christ, has hardly up to the present time existed, and does not to-day exist. They consider that this unity may indeed be desired and that it may even be one day attained through the instrumentality of wills directed to a common end, but that meanwhile it can only be regarded as mere ideal. They add that the Church in itself, or of its nature, is divided into sections; that is to say, that it is made up of several churches or distinct communities, which still remain separate, and although having certain articles of doctrine in common, nevertheless disagree concerning the remainder; that these all enjoy the same rights; and that the Church was one and unique from, at the most, the apostolic age until the first Ecumenical Councils. Controversies therefore, they say, and longstanding differences of opinion which keep asunder till the present day the members of the Christian family, must be entirely put aside, and from the remaining doctrines a common form of faith drawn up and proposed for belief, and in the profession of which all may not only know but feel that they are brothers. The manifold churches or communities, if united in some kind of universal federation, would then be in a position to oppose strongly and with success the progress of irreligion. This, Venerable Brethren, is what is commonly said. There are some, indeed, who recognize and affirm that Protestantism, as they call it, has rejected, with a great lack of consideration, certain articles of faith and some external ceremonies, which are, in fact, pleasing and useful, and which the Roman Church still retains. They soon, however, go on to say that that Church also has erred, and corrupted the original religion by adding and proposing for belief certain doctrines which are not only alien to the Gospel, but even repugnant to it. Among the chief of these they number that which concerns the primacy of jurisdiction, which was granted to Peter and to his successors in the See of Rome. Among them there indeed are some, though few, who grant to the Roman Pontiff a primacy of honor or even a certain jurisdiction or power, but this, however, they consider not to arise from the divine law but from the consent of the faithful. Others again, even go so far as to wish the Pontiff Himself to preside over their motley, so to say, assemblies. But, all the same, although many non-Catholics may be found who loudly preach fraternal communion in Christ Jesus, yet you will find none at all to whom it ever occurs to submit to and obey the Vicar of Jesus Christ either in His capacity as a teacher or as a governor. Meanwhile they affirm that they would willingly treat with the Church of Rome, but on equal terms, that is as equals with an equal: but even if they could so act. it does not seem open to doubt that any pact into which they might enter would not compel them to turn from those opinions which are still the reason why they err and stray from the one fold of Christ.
This being so, it is clear that the Apostolic See cannot on any terms take part in their assemblies, nor is it anyway lawful for Catholics either to support or to work for such enterprises; for if they do so they will be giving countenance to a false Christianity, quite alien to the one Church of Christ. 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. Jesus Christ sent His Apostles into the whole world in order that they might permeate all nations with the Gospel faith, and, lest they should err, He willed beforehand that they should be taught by the Holy Ghost: has then this doctrine of the Apostles completely vanished away, or sometimes been obscured, in the Church, whose ruler and defense is God Himself? 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. But the Only-begotten Son of God, when He commanded His representatives to teach all nations, obliged all men to give credence to whatever was made known to them by "witnesses preordained by God," and also confirmed His command with this sanction: "He that believeth and is baptized shall be saved; but he that believeth not shall be condemned." These two commands of Christ, which must be fulfilled, the one, namely, to teach, and the other to believe, cannot even be understood, unless the Church proposes a complete and easily understood teaching, and is immune when it thus teaches from all danger of erring. In this matter, those also turn aside from the right path, who think that the deposit of truth such laborious trouble, and with such lengthy study and discussion, that a man's life would hardly suffice to find and take possession of it; as if the most merciful God had spoken through the prophets and His Only-begotten Son merely in order that a few, and those stricken in years, should learn what He had revealed through them, and not that He might inculcate a doctrine of faith and morals, by which man should be guided through the whole course of his moral life.
These pan-Christians who turn their minds to uniting the churches seem, indeed, to pursue the noblest of ideas in promoting charity among all Christians: nevertheless how does it happen that this charity tends to injure faith? Everyone knows that John himself, the Apostle of love, who seems to reveal in his Gospel the secrets of the Sacred Heart of Jesus, and who never ceased to impress on the memories of his followers the new commandment "Love one another," altogether forbade any intercourse with those who professed a mutilated and corrupt version of Christ's teaching: "If any man come to you and bring not this doctrine, receive him not into the house nor say to him: God speed you." For which reason, since charity is based on a complete and sincere faith, the disciples of Christ must be united principally by the bond of one faith. Who then can conceive a Christian Federation, the members of which retain each his own opinions and private judgment, even in matters which concern the object of faith, even though they be repugnant to the opinions of the rest? And in what manner, We ask, can men who follow contrary opinions, belong to one and the same Federation of the faithful? For example, those who affirm, and those who deny that sacred Tradition is a true fount of divine Revelation; those who hold that an ecclesiastical hierarchy, made up of bishops, priests and ministers, has been divinely constituted, and those who assert that it has been brought in little by little in accordance with the conditions of the time; those who adore Christ really present in the Most Holy Eucharist through that marvelous conversion of the bread and wine, which is called transubstantiation, and those who affirm that Christ is present only by faith or by the signification and virtue of the Sacrament; those who in the Eucharist recognize the nature both of a sacrament and of a sacrifice, and those who say that it is nothing more than the memorial or commemoration of the Lord's Supper; those who believe it to be good and useful to invoke by prayer the Saints reigning with Christ, especially Mary the Mother of God, and to venerate their images, and those who urge that such a veneration is not to be made use of, for it is contrary to the honor due to Jesus Christ, "the one mediator of God and men." How so great a variety of opinions can make the way clear to effect the unity of the Church We know not; that unity can only arise from one teaching authority, one law of belief and one faith of Christians. But We do know that from this it is an easy step to the neglect of religion or indifferentism and to modernism, as they call it. Those, who are unhappily infected with these errors, hold that dogmatic truth is not absolute but relative, that is, it agrees with the varying necessities of time and place and with the varying tendencies of the mind, since it is not contained in immutable revelation, but is capable of being accommodated to human life. Besides this, in connection with things which must be believed, it is nowise licit to use that distinction which some have seen fit to introduce between those articles of faith which are fundamental and those which are not fundamental, as they say, as if the former are to be accepted by all, while the latter may be left to the free assent of the faithful: for the supernatural virtue of faith has a formal cause, namely the authority of God revealing, and this is patient of no such distinction. For this reason it is that all who are truly Christ's believe, for example, the Conception of the Mother of God without stain of original sin with the same faith as they believe the mystery of the August Trinity, and the Incarnation of our Lord just as they do the infallible teaching authority of the Roman Pontiff, according to the sense in which it was defined by the Ecumenical Council of the Vatican. Are these truths not equally certain, or not equally to be believed, because the Church has solemnly sanctioned and defined them, some in one age and some in another, even in those times immediately before our own? Has not God revealed them all? For the teaching authority of the Church, which in the divine wisdom was constituted on earth in order that revealed doctrines might remain intact for ever, and that they might be brought with ease and security to the knowledge of men, and which is daily exercised through the Roman Pontiff and the Bishops who are in communion with him, has also the office of defining, when it sees fit, any truth with solemn rites and decrees, whenever this is necessary either to oppose the errors or the attacks of heretics, or more clearly and in greater detail to stamp the minds of the faithful with the articles of sacred doctrine which have been explained. But in the use of this extraordinary teaching authority no newly invented matter is brought in, nor is anything new added to the number of those truths which are at least implicitly contained in the deposit of Revelation, divinely handed down to the Church: only those which are made clear which perhaps may still seem obscure to some, or that which some have previously called into question is declared to be of faith. (Papal Encyclical, Mortalium Animos, 6 January 1928).