Mortalium Animos
by Pope Pius XI
These... who strive for the union of the churches would appear to pursue the noblest of ideals in promoting charity among all Christians. But... John himself, the Apostle of love, strictly 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� (John 2:10). ...it is clear why this Apostolic See has never allowed its subjects to take part in the assemblies of non-Catholics. There is but one way in which the unity of Christians may be fostered, and that is by furthering the return to the one true Church of Christ of those who are separated from it... it is chiefly by the bond of one Faith that the disciples of Christ are to be united.

Cantate Domino
by Pope Eugene IV
The most Holy Roman Church firmly believes, professes, and preaches, that none of those existing outside the Catholic Church, not only pagans, but also Jews arid heretics and schismatics, can have a share in life eternal; but that they will go into the eternal fire which was prepared for the devil and his angels, unless before death they are joined with her; and that so important is the unity of this ecclesiastical body that only those remaining within this unity can profit by the sacraments of the Church unto salvation, and they alone can receive an eternal recompense for their fasts, their almsgiving, their other works of Christian piety and the duties of a Christian soldier. No one, let his almsgiving be as great as it may, no one, even if he pour out his blood for the Name of Christ, can be saved, unless he remain within the bosom and unity of the Catholic Church.

Fourth Lateran Council
There is but one universal Church of the faithful, outside of which no one at all can be saved.

1917 Code of Canon Law
Canon 1101
It is unlawful for the faithful to assist in any active manner, or to take part, in the services of non-Catholics.

Satis Cognitum
by Pope Leo XIII
There must also be the fitting and devout worship of God, which is to be found chiefly in the divine Sacrifice and in the dispensation of the sacraments, as well as salutary laws and discipline. All these must be found in the Church, since it continues the mission of the Savior forever. The Church alone offers to the human race that religion � that state of absolute perfection � which He wished, as it were to be incorporated in it. And it alone supplies those means of salvation which accord with the ordinary counsels of Providence.