76 Pusillus of Lamasba said: "I believe that baptism is not unto salvation except within the Catholic Church. Whatsoever is without the Catholic Church is mere pretense."
77. This indeed is true, that "baptism is not unto salvation except within the Catholic Church." For in itself it can indeed exist outside the Catholic Church as well; but there it is not unto salvation, because there it does not work salvation; just as that sweet savor of Christ is certainly not unto salvation in them that perish, though from a fault not in itself, but in them. But "whatsoever is without the Catholic Church is mere pretense," yet only in so far as it is not Catholic. But there may be something Catholic outside the Catholic Church, just as the name of Christ could exist outside the congregation of Christ, in which name he who did not follow with the disciples was casting out devils. For there may be pretense also within the Catholic Church, as is unquestionable in the case of those "who renounce the world in words and not in deeds," and yet the pretense is not Catholic. As, therefore, there is in the Catholic Church something which is not Catholic, so there may be something which is Catholic outside the Catholic Church.
Source: On Baptism, Against the Donatists (New Advent)