4 Satius of Sicilibba said: "If heretics receive forgiveness of their sins in their own baptism, it is without reason that they come to the Church. For since it is for sins that men are punished in the day of judgment, heretics have nothing to fear in the judgment of Christ if they have obtained remission of their sins."
5. This too might also have been our own judgment; but let its author beware in what spirit it was said. For it is expressed in terms of such import, that I should feel no compunction in consenting and subscribing to it in the same spirit in which I too believe that heretics may indeed have the baptism of Christ, but cannot have the remission of their sins. But he does not say, If heretics baptize or are baptized, but "If heretics," he says, "receive forgiveness of their sins in their own baptism, it is without reason that they come to the Church." For if we were to set in the place of heretics those whom Cyprian knew within the Church as "renouncing the world in words alone and not in deeds," we also might express this same judgment, in just so many words, with the most perfect truth. If those who only seem to be converted receive forgiveness of their sins in their own baptism, it is without reason that they are afterwards led on to a true conversion. For since it is for sins that men are punished in the day of judgment, "those who renounce the world in words and not in deeds" have nothing to fear in the judgment of Christ if they have obtained remission of their sins. But this reasoning is only made perfect by some such context as is formed by the addition of the words. But they ought to fear the judgment of Christ, and to lose no time in being converted in the truth of their hearts; and, when they have done this, it is certainly not necessary that they should be baptized a second time. It was possible, therefore, for them to receive baptism, and either not to receive remission of their sins, or to be burdened again at once with the load of sins which were forgiven them; and so the same is the case also with the heretics.
Source: On Baptism, Against the Donatists (New Advent)