6 Our opponents may perhaps ask us to point out passages where Christ is predicted by the prophets of Israel. One would think they might be satisfied with the authority of the apostles, who declare that what we read in the writings of the Hebrew prophets was fulfilled in Christ, or with that of Christ Himself, who says that these things were written of Him. Whoever is unable to point out the passages should lay the blame on his own ignorance; for the apostles and Christ and the sacred Scriptures are not chargeable with falsehood.
However, one instance out of many may be adduced. The apostle, in the verses following the passage quoted above, says: "The word of God cannot fail. For they are not all Israel which are of Israel; neither, because they are the seed of Abraham, are they all children: but, In Isaac shall your seed be called: that is, they which are the children of the flesh, these are not the children of God; but the children of promise are counted for the seed." What can our opponent say against this, in view of the declaration made to Abraham: "In your seed shall all the nations of the earth be blessed?"
At the time when the apostle gave the following exposition of this promise, "To Abraham and to his seed were the promises made. He says not, To seed, as of many, but as of one, To your seed, which is Christ," a doubt on this point might then have been less inexcusable, for at that time all nations had not yet believed on Christ, who is preached as of the seed of Abraham. But now that we see the fulfillment of what we read in the ancient prophecy—now that all nations are actually blessed in the seed of Abraham, to whom it was said thousands of years ago, "In your seed shall all nations be blessed,"— it is mere obstinate folly to try to bring in another Christ, not of the seed of Abraham, or to hold that there are no predictions of Christ in the prophetical books of the children of Abraham.
Source: Reply to Faustus the Manichaean (New Advent)