2 But now that we have begun a disputation with a man of peace like Cyprian, let us go on. For when he had brought an objection against himself, which he knew was urged by his brethren, "What then will become of those who in times past, coming to the Church from heresy, were admitted without baptism? The Lord," he answers, "is able of His mercy to grant indulgence, and not to separate from the gifts of His Church those who, being admitted in all honesty to His Church, have fallen asleep within the Church." Well indeed has he assumed that charity can cover the multitude of sins.
But if they really had baptism, and this were not rightly perceived by those who thought that they should be baptized again, that error was covered by the charity of unity so long as it contained, not the discord and spirit of the devil, but merely human infirmity, until, as the apostle says, "if they were otherwise minded, the Lord should reveal it to them." But woe unto those who, being torn asunder from unity by a sacrilegious rupture, either rebaptize, if baptism exists with both us and them, or do not baptize at all, if baptism exist in the Catholic Church only.
Whether, therefore, they rebaptize, or fail to baptize, they are not in the bond of peace; wherefore let them apply a remedy to which they please of these two wounds. But if we admit to the Church without baptism, we are of the number of those who, as Cyprian has assumed, may receive pardon because they preserved unity. But if (as is, I think, already clear from what has been said in the earlier books) Christian baptism can preserve its integrity even amid the perversity of heretics, then even though any in those times did rebaptize, yet without departing from the bond of unity, they might still attain to pardon in virtue of that same love of peace, through which Cyprian bears witness that those admitted even without baptism might obtain that they should not be separated from the gifts of the Church.
Further, if it is true that with heretics and schismatics the baptism of Christ does not exist, how much less could the sins of others hurt those who were fixed in unity, if even men's own sins were forgiven when they came to it even without baptism! For if, according to Cyprian, the bond of unity is of such efficacy, how could they be hurt by other men's sins, who were unwilling to separate themselves from unity, if even the unbaptized, who wished to come to it from heresy, thereby escaped the destruction due to their own sins?
Source: On Baptism, Against the Donatists (New Advent)