24 Some one says, Does it then make no difference, if two men, rooted in like error and wickedness, be baptized without change of life or heart, one without, the other within the Church? I acknowledge that there is a difference. For he is worse who is baptized without, in addition to his other sin—not because of his baptism, however, but because he is without; for the evil of division is in itself far from insignificant or trivial. Yet the difference exists only if he who is baptized within has desired to be within not for the sake of any earthly or temporal advantage, but because he has preferred the unity of the Church spread throughout the world to the divisions of schism; otherwise he too must be considered among those who are without.
Let us therefore put the two cases in this way. Let us suppose that the one, for the sake of argument, held the same opinions as Photinus about Christ, and was baptized in his heresy outside the communion of the Catholic Church; and that another held the same opinion but was baptized in the Catholic Church, believing that his view was really the Catholic faith. I consider him as not yet a heretic, unless, when the doctrine of the Catholic faith is made clear to him, he chooses to resist it, and prefers that which he already holds; and till this is the case, it is clear that he who was baptized outside is the worse.
And so in the one case erroneous opinion alone, in the other the sin of schism also, requires correction; but in neither of them is the truth of the sacrament to be repeated. But if any one holds the same view as the first, and knows that it is only in heresy severed from the Church that such a view is taught or learned, but yet for the sake of some temporal emolument has desired to be baptized in the Catholic unity, or, having been already baptized in it, is unwilling on account of the said emolument to secede from it, he is not only to be considered as seceding, but his offense is aggravated, in so far as to the error of heresy and the division of unity he adds the deceit of hypocrisy.
Wherefore the depravity of each man, in proportion as it is more dangerous and wanting in straightforwardness, must be corrected with the more earnestness and energy; and yet, if he has anything that is good in him, especially if it be not of himself, but from God, we ought not to think it of no value because of his depravity, or to be blamed like it, or to be ascribed to it, rather than to His bountiful goodness, who even to a soul that plays the harlot, and goes after her lovers, yet gives His bread, and His wine, and His oil, and other food or ornaments, which are neither from herself nor from her lovers, but from Him who in compassion for her is even desirous to warn her to whom she should return.
Source: On Baptism, Against the Donatists (New Advent)