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. 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.
Source: Mortalium Animos (Vatican.va)