10 Petilianus said: "For everything consists of an origin and root; and if it have not something for a head, it is nothing: nor does anything well receive second birth, unless it be born again of good seed."
11. Augustine answered: Why will you put yourself forward in the room of Christ, when you will not place yourself under Him? He is the origin, and root, and head of him who is being born, and in Him we feel no fear, as we must in any man, whoever he may be, lest he should prove to be false and of abandoned character, and we should be found to be sprung from an abandoned source, growing from an abandoned root, united to an abandoned head. For what man can feel secure about a man, when it is written, "Cursed be the man that trusts in man?" But the seed of which we are born again is the word of God, that is, the gospel. Whence the apostle says, "For in Christ Jesus I have begotten you through the gospel." And yet he allows even those to preach the gospel who were preaching it not in purity, and rejoices in their preaching; because, although they were preaching it not in purity, but seeking their own, not the things which are Jesus Christ's, yet the gospel which they preached was pure. And the Lord had said of certain of like character, "Whatsoever they bid you observe, that observe and do; but do not yet after their works: for they say, and do not." If, therefore, what is in itself pure is preached in purity, then the preacher himself also, in that he is a partner with the word, has his share in begetting the believer; but if he himself be not regenerate, and yet what he preaches be pure, then the believer is born not from the barrenness of the minister but from the fruitfulness of the word.
Source: Answer to Letters of Petilian, Bishop of Cirta (New Advent)