Wherefore, since it is manifest that the baptism remains in the baptized person when he is separated from the Church, the baptism which is in him is certainly separated with him. And therefore not all who retain the baptism retain the Church, just as not all who retain the Church retain eternal life. Or if we say that only those retain the Church who observe the commandments of God, we at once concede that there are many who retain baptism, and do not retain the Church.
21. Therefore the heretic is not "the first to seize baptism," since he has received it from the Church. Nor, though he seceded, could baptism have been lost by him whom we assert no longer to retain the Church, and yet allow to retain baptism. Nor does any one "yield his birthright, and give it to a heretic," because he says that he took away with him what he could not give lawfully, but what would yet be according to law when given; or that he no longer has lawfully what yet is in accordance with law in his possession. But the birthright rests only in a holy conversation and good life, to which all belong of whom that bride consists as her members which has no spot or wrinkle, or that dove that groans amid the wickedness of the many crows—unless it be that, while Esau lost his birthright from his lust after a mess of pottage, we are yet to hold that it is retained by defrauders, robbers, usurers, envious persons, drunkards and the like, over whose existence in the Church of his time Cyprian groaned in his epistles. Wherefore, either it is not the same thing to retain the Church and to retain the birthright in divine things, or, if every one who retains the Church also retains the birthright, then all those wicked ones do not retain the Church who yet both seem and are allowed by every one of us to give baptism within the Church; for no one, save the man who is wholly ignorant of sacred things, would say that they retain the birthright in sacred things.
Source: On Baptism, Against the Donatists (New Advent)