4 But since now, as I said before, we have begun a disputation with the epistles of Cyprian, I think that I should not seem even to him, if he were present, "to be contending obstinately and persistently in defense of heretics against my brethren and my fellow priests," when he learned the powerful reasons which move us to believe that even among heretics, who are perversely obstinate in their malignant error, the baptism of Christ is yet in itself most holy, and most highly to be reverenced.
And seeing that he himself, whose testimony has such weight with us, bears witness that they were wont in past times to be admitted without a second baptism, I would have any one, who is induced by Cyprian's arguments to hold it as certain that heretics ought to be baptized afresh, yet consider that those who, on account of weight of the arguments on the other side, are not as yet persuaded that this should be so, hold the same place as those in past time, who in all honesty admitted men who were baptized in heresy on the simple correction of their individual error, and who were capable of salvation with them in virtue of the bond of unity.
And let any one, who is led by the past custom of the Church, and by the subsequent authority of a plenary Council, and by so many powerful proofs from holy Scripture, and by much evidence from Cyprian himself, and by the clear reasoning of truth, to understand that the baptism of Christ, consecrated in the words of the gospel, cannot be perverted by the error of any man on earth—let such an one understand, that they who then thought otherwise, but yet preserved their charity, can be saved by the same bond of unity.
And herein he should also understand of those who, in the society of the Church dispersed throughout the world, could not have been defiled by any tares, by any chaff, so long as they themselves desired to be fruitful grain, and who therefore severed themselves from the same bond of unity without any cause for the divorce, that at any rate, whichever of the two opinions be true—that which Cyprian then held, or that which was maintained by the universal voice of the Catholic Church, which Cyprian did not abandon,— in either case they, having most openly placed themselves outside in the plain sacrilege of schism, cannot possibly be saved, and all that they possess of the holy sacraments, and of the free gifts of the one legitimate Bridegroom, is of avail, while they continue what they are, for their confusion rather than the salvation of their souls.
Source: On Baptism, Against the Donatists (New Advent)