3 And so the Donatists in some matters are with us; in some matters have gone out from us. Accordingly, those things wherein they agree with us we do not forbid them to do; but in those things in which they differ from us, we earnestly encourage them to come and receive them from us, or return and recover them, as the case may be; and with whatever means we can, we lovingly busy ourselves, that they, freed from faults and corrected, may choose this course. We do not therefore say to them, "Abstain from giving baptism," but "Abstain from giving it in schism."
Nor do we say to those whom we see them on the point of baptizing, "Do not receive the baptism," but "Do not receive it in schism." For if any one were compelled by urgent necessity, being unable to find a Catholic from whom to receive baptism, and so, while preserving Catholic peace in his heart, should receive from one without the pale of Catholic unity the sacrament which he was intending to receive within its pale, this man, should he immediately depart this life, we deem to be none other than a Catholic.
But if he should be delivered from the death of the body, on his restoring himself in bodily presence to that Catholic congregation from which in heart he had never departed, so far from blaming his conduct, we should praise it with the greatest truth and confidence; because he trusted that God was present to his heart, while he was striving to preserve unity, and was unwilling to depart this life without the sacrament of holy baptism, which he knew to be of God, and not of men; wherever he might find it.
But if any one who has it in his power to receive baptism within the Catholic Church prefers, from some perversity of mind, to be baptized in schism, even if he afterwards bethinks himself to come to the Catholic Church, because he is assured that there that sacrament will profit him, which can indeed be received but cannot profit elsewhere, beyond all question he is perverse, and guilty of sin, and that the more flagrant in proportion as it was committed wilfully. For that he entertains no doubt that the sacrament is rightly received in the Church, is proved by his conviction that it is there that he must look for profit even from what he has received elsewhere.
Source: On Baptism, Against the Donatists (New Advent)