3 Christ as foretold by the Hebrew prophets does not please you; but this is the Christ in whom the Gentile nations believe, with whom, according to you, Hebrew prophecy should have no weight. They receive the gospel which, as Paul says, "God had promised before by His prophets in the Holy Scriptures of His Son, who was made of the seed of David according to the flesh." So we read in Isaiah: "There shall be a Root of Jesse, which shall rise to reign in the nations; in Him shall the Gentiles trust." And again: "Behold, a virgin shall conceive and bear a son, and they shall call His name Emmanuel," which is, being interpreted, God with us.
Nor let the Manichæan think that Christ is foretold only as a man by the Hebrew prophets; for this is what Faustus seems to insinuate when he says, "Our Christ is the Son of God," as if the Christ of the Hebrews was not the Son of God. We can prove Christ the virgin's son of Hebrew prophecy to be God. For the Lord Himself teaches the carnal Jews not to think that, because He is foretold as the son of David, He is therefore no more than that. He asks: "What think ye of Christ? Whose son is He?"
They reply: "Of David." Then, to remind them of the name Emmanuel, God with us, He says: "How does David in the Spirit call Him Lord, saying, The Lord said unto my Lord, Sit at my right hand, till I make Your enemies Your footstool?" Here, then, Christ appears as God in Hebrew prophecy. What prophecy can the Manichæans show with the name of Christ in it?
Source: Reply to Faustus the Manichaean (New Advent)