To help you to recover after the orgy of crowd-pleasing peace slogans – and worse, as I might write separately – some points from Pius XI’s Mortalium Animos.
All emphases mine. Enjoy:
…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.
8. 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.
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.
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.
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.”
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.
So, Venerable Brethren, it is clear why this Apostolic See has never allowed its subjects to take part in the assemblies of non-Catholics: for the union of Christians can only be promoted by promoting the return to the one true Church of Christ of those who are separated from it, for in the past they have unhappily left it.