8 Aurelius of Utica said: "Since the apostle says that we ought not to be partakers with the sins of other men, what else does he do but make himself partaker with the sins of other men, who holds communion with heretics without the baptism of the Church? And therefore I pronounce my judgment that heretics should be baptized, that they may receive remission of their sins, and so communion be allowed to them."
9. The answer is: Therefore Cyprian and all those bishops were partakers in the sins of other men, inasmuch as they remained in communion with such men, when they removed no one from the right of communion who entertained a different opinion. Where, then, is the Church? Then, to say nothing for the moment of heretics—since the words of this judgment are applicable also to other sinners, such as Cyprian saw with lamentation to be in the Church with him, whom, while he confuted them, he yet tolerated—where is the Church, which, according to these words must be held to have perished from that very moment by the contagion of their sins? But if, as is the most firmly established truth, the Church both has remained and does remain, the partaking of the sins of others, which is forbidden by the apostle, must be considered only to consist in consenting to them. But let heretics be baptized again, that they may receive remission of their sins, if the wayward and the envious are baptized again, who, seeing that "they renounced the world in words and not in deeds," were indeed able to receive baptism, but did not obtain remission of their sins, as the Lord says, "If you forgive not men their trespasses, neither will your Father forgive your trespasses."
Source: On Baptism, Against the Donatists (New Advent)