5 Wherefore, if they were in error, and would have perished had they not been corrected, who wished to be of Paul, what must we suppose to be the hope of those who wished to be of Donatus? For they use their utmost endeavors to prove that the origin, root, and head of the baptized person is none other than the individual by whom he is baptized. The result is, that since it is very often a matter of uncertainty what kind of man the baptizer is, the hope therefore of the baptized being of uncertain origin, of uncertain root, of uncertain head, is of itself uncertain altogether.
And since it is possible that the conscience of the giver may be in such a condition as to be accursed and defiled without the knowledge of the recipient, it results that, being of an accursed origin, accursed root, accursed head, the hope of the baptized may prove to be vain and ungrounded. For Petilian expressly states in his epistle, that "everything consists of an origin and root; and if it have not something for a head, it is nothing." And since by the origin and root and head of the baptized person he wishes to be understood the man by whom he is baptized, what good does the unhappy recipient derive from the fact that he does not know how bad a man his baptizer really is?
For he does not know that he himself has a bad head, or actually no head at all. And yet what hope can a man have, who, whether he is aware of it or not, has either a very bad head or no head at all? Can we maintain that his very ignorance forms a head, when his baptizer is either a bad head or none at all? Surely any one who thinks this is unmistakeably without a head.
Source: Answer to Letters of Petilian, Bishop of Cirta (New Advent)