96 Cyprian of Carthage said: "My opinion has been set forth with the greatest fullness in the letter which has been written to our colleague Jubaianus, that heretics being called enemies of Christ and antichrists according to the testimony of the gospel and the apostles, should, when they come to the Church, be baptized with the one baptism of the Church, that from enemies they may be made friends, and that from antichrists they may be made Christians."
97. What need is there of further disputation here, seeing that we have already handled with the utmost care that very epistle to Jubaianus of which he has made mention? And as to what he has said here, let us not forget that it might be said of all unrighteous men who, as he himself bears witness, are in the Catholic Church, and whose power of possessing and of conferring baptism is not questioned by any of us. For they come to the Church, who pass to Christ from the party of the devil, and build upon the rock, and are incorporated with the Dove, and are placed in security in the garden enclosed and fountain sealed; where none of those are found who live contrary to the precepts of Christ, wherever they may seem to be. For in the epistle which he wrote to Magnus, while discussing this very question, he himself warned us at sufficient length, and in no ambiguous terms, of what kind of society we should understand that the Church consists. For he says, in speaking of a certain man, "Let him become an alien and profane, an enemy to the peace and unity of the Lord, not dwelling in the house of God, that is to say, in the Church of Christ, in which none dwell save those who are of one heart and of one mind." Let those, therefore, who would lay injunctions on us on the authority of Cyprian, pay attention for a time to what we here say. For if only those who are of one heart and of one mind dwell in the Church of Christ, beyond all question those were not dwelling in the Church of Christ, however much they might appear to be within, who of envy and contention were announcing Christ without charity; by whom he understands, not the heretics and schismatics who are mentioned by the Apostle Paul, but false brethren holding conversation with him within, who certainly ought not to have baptized, because they were not dwelling in the Church, in which he himself says that none dwell save those who are of one heart and of one mind: unless, indeed, any one be so far removed from the truth as to say that those were of one heart and of one mind who were envious and malevolent, and contentious without charity; and yet they used to baptize: nor did the detestable waywardness which they displayed in any degree violate or diminish from the sacrament of Christ, which was handled and dispensed by them.
Source: On Baptism, Against the Donatists (New Advent)