103 But now I think that it is fully time for me to bring to their due termination these books also on the subject of baptism, in which our Lord God has shown to us, through the words of the peaceful Bishop Cyprian and his brethren who agreed with him, how great is the love which should be felt for catholic unity; so that even where they were otherwise minded until God should reveal even this to them, they should rather bear with those who thought differently from themselves, than sever themselves from them by a wicked schism; whereby the mouths of the Donatists are wholly closed, even if we say nothing of the followers of Maximian.
For if the wicked pollute the good in unity, then even Cyprian himself already found no Church to which he could be joined. But if the wicked do not infect the good in unity, then the sacrilegious Donatist has no ground to set before himself for separation. But if baptism is both possessed and transferred by the multitude of others who work the works of the flesh, of which it is said, that "they which do such things shall not inherit the kingdom of God," then it is possessed and transferred also by heretics, who are numbered among those works; because they could have transferred it had they remained, and did not lose it by their secession.
But men of this kind confer it on their fellows as fruitlessly and uselessly as the others who resemble them, inasmuch as they shall not inherit the kingdom of God. And as, when those others are brought into the right path, it is not that baptism begins to be present, having been absent before, but that it begins to profit them, having been already in them; so is it the case with heretics as well. Whence Cyprian and those who thought with him could not impose limits on the Catholic Church, which they would not mutilate. But in that they were otherwise minded we feel no fear, seeing that we too share in their veneration for Peter; yet in that they did not depart from unity we rejoice, seeing that we, like them, are founded on the rock.
Source: On Baptism, Against the Donatists (New Advent)