ONLY ONE FAITH IS TRUE

Pope Gregory XVI: 

 Now We consider another abundant source of the evils with which the Church is afflicted at present: indifferentism. This perverse opinion is spread on all sides by the fraud of the wicked who claim that it is possible to obtain the eternal salvation of the soul by the profession of any kind of religion, as long as morality is maintained. [...] With the admonition of the apostle that “there is one God, one faith, one baptism” may those fear who contrive the notion that the safe harbor of salvation is open to persons of any religion whatever. They should consider the testimony of Christ Himself that “those who are not with Christ are against Him,” 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.” 

Encyclical Mirari vos, N. 9, August 15, 1832.