28 But on the question whether we ought to prefer a Catholic of the most abandoned character to a heretic in whose life, except that he is a heretic, men can find nothing to blame, I do not venture to give a hasty judgment. But if any one says, because he is a heretic, he cannot be this only without other vices also following—for he is carnal and natural, and therefore must be also envious, and hot-tempered, and jealous, and hostile to truth itself, and utterly estranged from it—let him fairly understand, that of those other faults of which he is supposed to have chosen some one less flagrant, a single one cannot exist by itself in any man, because he in turn is carnal and natural; as, to take the case of drunkenness, which people have now become accustomed to talk of not only without horror, but with some degree of merriment, can it possibly exist alone in any one in whom it is found?
For what drunkard is not also contentious, and hot-tempered, and jealous, and at variance with all soundness of counsel, and at grievous enmity with those who rebuke him? Further, it is not easy for him to avoid being a fornicator and adulterer, though he may be no heretic; just as a heretic may be no drunkard, nor adulterer, nor fornicator, nor lascivious, nor a lover of money, or given to witchcraft, and cannot well be all these together. Nor indeed is any one vice followed by all the rest.
Supposing, therefore, two men,— one a Catholic with all these vices, the other a heretic free from all from which a heretic can be free—although they do not both contend against the faith, and yet each lives contrary to the faith, and each is deceived by a vain hope, and each is far removed from charity of spirit, and therefore each is severed from connection with the body of the one dove; why do we recognise in one of them the sacrament of Christ, and not in the other, as though it belonged to this or that man, while really it is the same in both, and belongs to God alone, and is good even in the worst of men?
And if of the men who have it, one is worse than another, it does not follow that the sacrament which they have is worse in the one than in the other, seeing that neither in the case of two bad Catholics, if one be worse than the other, does he possess a worse baptism, nor, if one of them be good and another bad, is baptism bad in the bad one and good in the good one; but it is good in both. Just as the light of the sun, or even of a lamp, is certainly not less brilliant when displayed to bad eyes than when seen by better ones; but it is the same in the case of both, although it either cheers or hurts them differently according to the difference of their powers.
Source: On Baptism, Against the Donatists (New Advent)