21 In view of the multitudes from all nations who have become zealous believers in these books, it is laughably absurd to tell us that it is impossible to persuade a Gentile to learn the Christian faith from Jewish books. Indeed, it is a great confirmation of our faith that such important testimony is borne by enemies. The believing Gentiles cannot suppose these testimonies to Christ to be recent forgeries; for they find them in books held sacred for so many ages by those who crucified Christ, and still regarded with the highest veneration by those who every day blaspheme Christ.
If the prophecies of Christ were the production of the preachers of Christ, we might suspect their genuineness. But now the preacher expounds the text of the blasphemer. In this way the Most High God orders the blindness of the ungodly for the profit of the saint, in His righteous government bringing good out of evil, that those who by their own choice live wickedly may be, in His just judgment, made the instruments of His will. So, lest those that were to preach Christ to the world should be thought to have forged the prophecies which speak of Christ as to be born, to work miracles, to suffer unjustly, to die, to rise again, to ascend to heaven, to publish the gospel of eternal life among all nations, the unbelief of the Jews has been made of signal benefit to us; so that those who do not receive in their heart for their own good these truths, carry, in their hands for our benefit the writings in which these truths are contained.
And the unbelief of the Jews increases rather than lessens the authority of the books, for this blindness is itself foretold. They testify to the truth by their not understanding it. By not understanding the books which predict that they would not understand, they prove these books to be true.
Source: Reply to Faustus the Manichaean (New Advent)