65 Quietus of Burug said: "We who live by faith ought with believing observance to obey what has been before foretold for our instruction. For it is written in Solomon, 'He that is washed by one dead, what avails his washing?' Which assuredly he says of those who are washed by heretics, and of those who wash. For if they who are baptized among them receive eternal life through the remission of their sins, why do they come to the Church? But if no salvation is received from a dead person, and they therefore, acknowledging their former error, return with penance to the truth, they ought to be sanctified with the one life-giving baptism which is in the Catholic Church."
66. What it is to be baptized by the dead, we have already, without prejudice to the more careful consideration of the same scripture, sufficiently declared before. But I would ask why it is that they wish heretics alone to be considered dead, when Paul the apostle has said generally of sin, "The wages of sin is death;" and again, "To be carnally minded is death." And when he says that a widow that lives in pleasure is dead, how are they not dead "who renounce the world in words and not in deeds"? What, therefore, is the profit of washing in him who is baptized by them, except, indeed, that if he himself also is of the same character, he has the laver indeed, but it does not profit him to salvation? But if he by whom he is baptized is such, but the man who is baptized is turned to the Lord with no false heart, he is not baptized by that dead person, but by that living One of whom it is said, "The same is He which baptizes." But to what he says of heretics, that if they who are baptized among them receive eternal life through the remission of their sins, why do they come to the Church? We answer: They come for this reason, that although they have received the baptism of Christ up to the point of the celebration of the sacrament, yet they cannot attain to life eternal save through the charity of unity; just as neither would those envious and malicious ones attain to life eternal, who would not have their sins forgiven them, even if they entertained hatred only against those from whom they suffered wrong; since the Truth said, "If you forgive not men their trespasses, neither will your Father forgive your trespasses," how much less when they were hating those towards whom they were rewarding evil for good? And yet these men, though "renouncing the world in words and not in deeds," would not be baptized again, if they should afterwards be corrected, but they would be made holy by the one living baptism. And this is indeed in the Catholic Church, but not in it alone, as neither is it in the saints alone who are built upon the rock, and of whom that one dove is composed.
Source: On Baptism, Against the Donatists (New Advent)