13 The elder Felix of Migirpa said: "I think that every one coming from heresy should be baptized. For in vain does any one suppose that he has been baptized there, seeing that there is no baptism save the one true baptism in the Church; for there is one Lord, and one faith, and one Church, in which rests the one baptism, and holiness, and the rest. For the things that are practised without have no power to work salvation."
14. To what Felix of Migirpa said we answer as follows. If the one true baptism did not exist except in the Church, it surely would not exist in those who depart from unity. But it does exist in them, since they do not receive it when they return, simply because they had not lost it when they departed. But as regards his statement, that "the things that are practised without have no power to work salvation," I agree with him, and think that it is quite true; for it is one thing that baptism should not be there, and another that it should have no power to work salvation. For when men come to the peace of the Catholic Church, then what was in them before they joined it, but did not profit them, begins at once to profit them.
Source: On Baptism, Against the Donatists (New Advent)