30 Accordingly we agree with Cyprian that "heretics cannot give remission of sins;" but we maintain that they can give baptism—which indeed in them, both when they give and when they receive it, is profitable only to their destruction, as misusing so great a gift of God; just as also the malicious and envious, whom Cyprian himself acknowledges to be within the Church, cannot give remission of sins, while we all confess that they can give baptism. For if it was said of those who have sinned against us, "If you forgive not men their trespasses, neither will your Father forgive your trespasses," how much more impossible is it that their sins should be forgiven who hate the brethren by whom they are loved, and are baptized in that very hatred; and yet when they are brought to the right way, baptism is not given them anew, but that very pardon which they did not then deserve is granted them in their true conversion?
And so even what Cyprian wrote to Quintus, and what, in conjunction with his colleagues Liberalis, Caldonius, Junius, and the rest, he wrote to Saturninus, Maximus, and others, is all found, on due consideration, to be in no wise meet to be preferred as against the agreement of the whole Catholic Church, of which they rejoiced that they were members, and from which they neither cut themselves away nor allowed others to be cut away who held a contrary opinion, until at length, by the will of the Lord, it was made manifest, by a plenary Council many years afterwards, what was the more perfect way, and that not by the institution of any novelty, but by confirming what was old.
Source: On Baptism, Against the Donatists (New Advent)