17 Faustus has not told us how he would prove the divinity of Christ to the heathen, whom he makes to say: I believe neither the prophets in support of Christ, nor Christ in support of the prophets. It would be absurd to suppose that such a man would believe what Christ says of Himself, when he disbelieves what He says of others. For if he thinks Him unworthy of credit in one case, he must think Him so in all, or at least more so when speaking of Himself than when speaking of others.
Perhaps, failing this, Faustus would read to him the Sibyls and Orpheus, and any heathen prophecies about Christ that he could find. But how could he do this, when he confesses that he knows none? His words are: "If, as is said, any prophecies of Christ are to be found in the Sibyl, or in Hermes, called Trismegistus, or Orpheus, or any heathen poet." How could he read writings of which he knows nothing, and which he supposes to exist only from report, to one who will not believe either the prophets or Christ?
What, then, would he do? Would he bring forward Manichæus as a witness to Christ? The opposite of this is what the Manichæans do. They take advantage of the widespread fragrance of the name of Christ to gain acceptance for Manichæus, that the edge of their poisoned cup may be sweetened with this honey. Taking hold of the promises of Christ to His disciples that He would send the Paraclete, that is, the Comforter or Advocate, they say that this Paraclete is Manichæus, or in Manichæus, and so steal an entrance into the minds of men who do not know when He who was promised by Christ really came.
Those who have read the canonical book called the Acts of the Apostles find a reference to Christ's promise, and an account of its fulfillment. Faustus, then, has no proof to give to the inquirer. It is not likely that any one will be so infatuated as to take the authority of Manichæus when he rejects that of Christ. Would he not reply in derision, if not in anger, Why do you ask me to believe Persian books, when you forbid me to believe Hebrew books? The Manichæan has no hold on the inquirer, unless he is already in some way convinced of the truth of Christianity.
When he finds him willing to believe Christ, then he deludes him with the representation of Christ given by Manichæus. So the partridge gathers what it has not brought forth. When will you whom he gathers leave him? When will you see him to be a fool, who tells you that Hebrew testimony is worthless in the case of unbelievers, and superfluous to believers?
Source: Reply to Faustus the Manichaean (New Advent)