34 I entreat of you, pay attention to this: I ask where the means shall be found for cleansing the conscience of the recipient, when he is not acquainted with the stain upon the conscience of him that gives but not in holiness, if the conscience of him that gives in holiness is waited for to cleanse the conscience of the recipient? And from what source he is to receive faith, who is unwittingly baptized by one that is faithless, if, whosoever has received his faith wittingly from one that is faithless, receives not faith but guilt?
And he answers me, that both the baptizer and the baptized should be subjected to examination. And for the proof of this point, out of which no question arises, he adduces the example of John, in that he was examined by those who asked him who he claimed to be, and that he also in turn examined those to whom he says, "O generation of vipers, who has warned you to flee from the wrath to come?" What has this to do with the subject? What has this to do with the question under discussion?
God had vouchsafed to John the testimony of most eminent holiness of life, confirmed by the previous witness of the noblest prophecy, both when he was conceived, and when he was born. But the Jews put their question, already believing him to be a saint, to find out which of the saints he maintained himself to be, or whether he was himself the saint of saints, that is, Christ Jesus. So much favor indeed was shown to him, that credence would at once have been given to whatever he might have said about himself.
If, therefore, we are to follow this precedent in declaring that each several baptizer is now to be examined, then each must also be believed, whatever he may say of himself. But who is there that is made up of deceit, whom we know that the Holy Spirit flees from, in accordance with the Scripture, who would not wish the best to be believed of him, or who would hesitate to bring this about by the use of any words within his reach? Accordingly, when he shall have been asked who he is, and shall have answered that he is the faithful dispenser of God's ordinances, and that his conscience is not polluted with the stain of any crime, will this be the whole examination, or will there be a further more careful investigation into his character and life?
Assuredly there will. But it is not written that this was done by those who in the desert of Jordan asked John who he was.
Source: Answer to Letters of Petilian, Bishop of Cirta (New Advent)