6 Victor of Gor said: "Seeing that sins are forgiven only in the baptism of the Church, he who admits heretics to communion without baptism is guilty of two errors contrary to reason; for, on the one hand, he does not cleanse the heretics, and, on the other, he defiles the Christians."
7. To this we answer that the baptism of the Church exists even among heretics, though they themselves are not within the Church; just as the water of Paradise was found in the land of Egypt, though that land was not itself in Paradise. We do not therefore admit heretics to communion without baptism; and since they come with their waywardness corrected, we receive not their sins, but the sacraments of Christ. And, in respect of the remission of their sins, we say again here exactly what we said above. And certainly, in regard of what he says at the end of his judgment, declaring that he "is guilty of two errors contrary to reason, seeing that on the one hand he does not cleanse the heretics, and on the other he defiles the Christians," Cyprian himself is the first and the most earnest in repudiating this with the colleagues who agreed with him. For neither did he think that he was defiled, when, on account of the bond of peace, he decreed that it was right to hold communion with such men, when he used the words, "Judging no one, nor removing any from the right of communion if he entertain a different opinion." Or, if heretics defile the Church by being admitted to communion without being baptized, then the whole Church has been defiled in virtue of that custom which has been so often recorded here. And just as those men call us traditors because of our forefathers, in whom they were able to prove nothing of the sort when they laid the charge against them, so, if every man partakes of the character of those with whom he may have held communion, all were then made heretics. And if every one who asserts this is mad, it must be false that Victor says, when he declares that "he who admits heretics to communion without baptism, not only fails to cleanse the heretics, but pollutes the Christians as well." Or if this be true, they were then not admitted without baptism, but those men had the baptism of Christ, although it was given and received among heretics, who were so admitted in accordance with that custom which these very men acknowledged to exist; and on the same grounds they are even now rightly admitted in the same manner.
Source: On Baptism, Against the Donatists (New Advent)