21 For it often happens that a man has an enemy whom he hates most unjustly; although we are commanded to love even our unjust enemies, and to pray for them. But in some sudden danger of death he begins to be uneasy, and desires baptism, which he receives in such haste, that the emergency scarcely admits of the necessary formal examination of a few words, much less of a long conversation, so that this hatred should be driven from his heart, even supposing it to be known to the minister who baptizes him.
Certainly cases of this sort are still found to occur not only with us, but also with them. What shall we say then? Are this man's sins forgiven or not? Let them choose just which alternative they prefer. For if they are forgiven, they immediately return: this is the teaching of the gospel, the authoritative announcement of truth. Whether, therefore, they are forgiven or not, medicine is necessary afterwards; and yet if the man lives, and learns that his fault stands in need of correction, and corrects it, he is not baptized anew, either with them or with us.
So in the points in which schismatics and heretics neither entertain different opinions nor observe different practice from ourselves, we do not correct them when they join us, but rather commend what we find in them. For where they do not differ from us, they are not separated from us. But because these things do them no good so long as they are schismatics or heretics, on account of other points in which they differ from us, not to mention the most grievous sin that is involved in separation itself, therefore, whether their sins remain in them, or return again immediately after remission, in either case we exhort them to come to the soundness of peace and Christian charity, not only that they may obtain something which they had not before, but also that what they had may begin to be of use to them.
Source: On Baptism, Against the Donatists (New Advent)