18 But as regards the remission of sins, whether it is granted through baptism at the hands of the heretics, I have already expressed my opinion on this point in a former book; but I will shortly recapitulate it here. If remission of sins is there conferred by the sacredness of baptism, the sins return again through obstinate perseverance in heresy or schism; and therefore such men must needs return to the peace of the Catholic Church, that they may cease to be heretics and schismatics, and deserve that those sins which had returned on them should be cleansed away by love working in the bond of unity.
But if, although among heretics and schismatics it be still the same baptism of Christ, it yet cannot work remission of sins owing to this same foulness of discord and wickedness of dissent, then the same baptism begins to be of avail for the remission of sins when they come to the peace of the Church—[not] that what has been already truly remitted should not be retained; nor that heretical baptism should be repudiated as belonging to a different religion, or as being different from our own, so that a second baptism should be administered; but that the very same baptism, which was working death by reason of discord outside the Church, may work salvation by reason of the peace within.
It was, in fact, the same savor of which the apostle says, "We are a sweet savor of Christ in every place;" and yet, says he, "both in them that are saved and in them that perish. To the one we are the savor of life unto life; and to the other the savor of death unto death." And although he used these words with reference to another subject, I have applied them to this, that men may understand that what is good may not only work life to those who use it aright, but also death to those who use it wrong.
Source: On Baptism, Against the Donatists (New Advent)