Beatus Vir

I'm reading (actually re-reading, since I don't remember reading it before, but have a lot of sections underlined) St. Augustine's On Baptism, Against the Donatists. In the rejection of Donatism, the Church insists that baptism is valid even when administered by wicked men, heretics, or traditores. Anyone who has ever heard of Donatism probably knows that. What they may not know, is that while a person may still possess or receive a true baptism while outside the pale of the Church, that baptism will be of no benefit to those who remain in separation from the Catholic Church. It is not so because of any deficency in baptism, rather the person cleansed from sin in Baptism, is made dirty again by virture of the schism, the person clothed in Christ is baptism, is striped of Him by schism, and the person enlightened by baptism, returned to darkness by virtue of the schism.

Anyway, here's the Bishop of Hippo:

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 forthwith 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." (Book I, Ch. 2)

...And so let them understand that men may be baptized in communions severed from the Church, in which Christ’s baptism is given and received in the said celebration of the sacrament, but that it will only then be of avail for the remission of sins, when the recipient, being reconciled to the unity of the Church, is purged from the sacrilege of deceit, by which his sins were retained, and their remission prevented. For, as in the case of him who had approached the sacrament in deceit there is no second baptism, but he is purged by faithful discipline and truthful confession, which he could not be without baptism, so that what was given before becomes then powerful to work his salvation, when the former deceit is done away by the truthful confession; so also in the case of the man who, while an enemy to the peace and love of Christ, received in any heresy or schism the baptism of Christ, which the schismatics in question had not lost from among them, though by his sacrilege his sins were not remitted, yet, when he corrects his error, and comes over to the communion and unity of the Church, he ought not to be again baptized: because by his very reconciliation to the peace of the Church he receives this benefit, that the sacrament now begins in unity to be of avail for the remission of his sins, which could not so avail him as received in schism. (Book 1, Ch. 12)