26 Faustus uses an argument which is either very deceitful or very stupid. And as Faustus is not stupid, it is probable that he used the argument intentionally, with the design of misleading the careless reader. He says: If these things are not written of Christ, and if you cannot show any others, it follows that there are none at all. The proposition is true; but it remains to be proved, both that these things are not written of Christ, and that no other can be shown.
Faustus has not proved this; for we have shown both how these things are to be understood of Christ, and that there are many other things which have no meaning but as applied to Christ. So it does not follow, as Faustus says, that nothing was written by Moses of Christ. Let us repeat Faustus' argument: If these things are not written of Christ, and if you cannot show any others, it follows that there are none at all. Perfectly so. But as both these things and many others have been shown to be written of Christ, or with reference to Christ, the true conclusion is that Faustus' argument is worthless.
In the passages quoted by Faustus, he has tried, though without success, to show that they were not written of Christ. But in order to draw the conclusion that there are none at all, he should first have proved that no others can be shown. Instead of this, he takes for granted that the readers of his book will be blind, or the hearers deaf, so that the omission will be overlooked, and runs on thus: If there are none, Christ could not have asserted that there were any. And if Christ did not make this assertion, it follows that this verse is spurious.
Here is a man who thinks so much of what he says himself, that he does not consider the possibility of another person saying the opposite. Where is your wit? Is this all you could say for a bad cause? But if the badness of the cause made you utter folly, the bad cause was your own choice. To prove your antecedent false, we have only to show some other things written of Christ. If there are some, it will not be true that there are none. And if there are some, Christ may have asserted that there were.
And if Christ may have asserted this, it follows that this verse of the Gospel is not spurious. Coming back, then, to Faustus' proposition, If you cannot show any other, it follows that there are none at all, it requires to be proved that we cannot show any other. We need only refer to what we showed before, as sufficient to prove the truth of the text in the Gospel, in which Christ says, "If you believed Moses, you would also believe me; for he wrote of me." And even though from dullness of mind we could find nothing written of Christ by Moses, still, so strong is the evidence in support of the authority of the Gospel, that it would be incumbent on us to believe that not only some things, but everything written by Moses, refers to Christ; for He says not, He wrote also of me, but, He wrote of me.
The truth then is this, that even though there were doubts, which God forbid, of the genuineness of this verse, the doubt would be removed by the number of testimonies to Christ which we find in Moses; while, on the other hand, even if we could find none, we should still be bound to believe that these are to be found, because no doubts can be admitted regarding any verse in the Gospel.
Source: Reply to Faustus the Manichaean (New Advent)