9 When he hears, "Every good tree brings good fruit, but a corrupt tree brings forth evil fruit: do men gather grapes of thorns?" and, "A good man out of the good treasure of his heart brings forth good things, and an evil man out of the evil treasure brings forth evil things;" he will answer, This therefore is good fruit, that I should be a good tree, that is, a good man, that I should show forth good fruit, that is, good works. But this will be given to me, not by him that plants, nor by him that waters, but by God that gives the increase. For if the good tree be the good baptizer, so that his good fruit should be the man whom he baptizes, then any one who has been baptized by a bad man, even if his wickedness be not manifest, will have no power to be good, for he is sprung from an evil tree. For a good tree is one thing; a tree whose quality is concealed, but yet bad, is another. Or if, when the tree is bad, but hides its badness, then whosoever is baptized by it is born not of it, but of Christ; then they are justified with more perfect holiness who are baptized by the bad who hide their evil nature, than they who are baptized by the manifestly good.
Source: Answer to Letters of Petilian, Bishop of Cirta (New Advent)