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Apostolic Letter of Pope Leo XIII, Praeclara Gratulationis Publicae (The Reunion of Christendom), June 20, 1894:
Nay, more, things have already come to such a pass that many do not even hesitate to root up the very Foundation upon which alone rests all Religion, and the hope of men, to wit, the Divine Nature of Jesus Christ, Our Savior. And again, whereas formerly they used to assert that the books of the Old and the New Testament were written under the inspiration of God, they now deny them that Authority; this, indeed, was an inevitable consequence when they granted to all the right of private interpretation. Hence, too, the acceptance of individual conscience as the sole guide and rule of conduct to the exclusion of any other: hence those conflicting opinions and numerous sects that fall away so often into the doctrines of Naturalism and Rationalism.
Therefore it is, that having lost all hope of an agreement in their persuasions, they now proclaim and recommend a union of brotherly love. And rightly, too, no doubt, for we should all be united by the bond of mutual Charity. Our Lord Jesus Christ enjoined it most emphatically, and wished that this love of one another should be the mark of His Disciples. But how can hearts be united in perfect Charity where minds do not agree in Faith?
Pope Pius XI, Mortalium Animos (# 2), Jan. 6, 1928: “For which reason conventions, meetings and addresses are frequently arranged by these persons, at which a large number of listeners are present, and at which all without distinction are invited to join in the discussion, both infidels of every kind, and Christians… Certainly such attempts can nowise be approved by Catholics, founded as they are on that false opinion which considers all religions to be more or less good and praiseworthy… Not only are those who hold this opinion in error and deceived, but also in distorting the idea of true religion they reject it…”
Pope Pius XI, Mortalium Animos #9, Jan. 6, 1928: “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 form 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.’ (II John 10).”
Pope Pius XI, Mortalium Animos (#10), Jan. 6, 1928: “… 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…”
Pope Gregory XVI, Mirari Vos (#13), Aug. 15, 1832: "They should consider the testimony of Christ Himself that 'those who are not with Christ are against Him,' (Lk 11:23) and that they disperse unhappily who do not gather with Him. Therefore, 'without a doubt they will perish forever, unless they hold the Catholic faith whole and inviolate' (Athanasian Creed)."