5 Augustine replied: The Catholic, which is also the apostolic, doctrine, is, that our Lord and Saviour Jesus Christ is both the Son of God in His divine nature, and the Son of David after the flesh. This we prove from the writings of the evangelists and apostles, so that no one can reject our proofs without also rejecting these writings. Faustus' plan is to represent some one as saying a few words, without bringing forward any evidence in answer to Faustus' fertile sophistry.
But with all his ingenuity, the proofs I have to give will leave Faustus no reply, but that these passages are spurious interpolations in the sacred record—a reply which serves as a means of escaping, or of trying to escape, the force of the plainest statements in Holy Scripture. We have already in this treatise sufficiently exposed the irrational absurdity, as well as the daring profanity, of such criticism; and not to exceed all limits, we must avoid repetition. It cannot be necessary that we should bring together all the passages scattered throughout Scripture, which show, in answer to Faustus, that in the books of the highest and most sacred authority He who is called the only-begotten Son of God, even God with God, is also called the Son of David, on account of His taking the form of a servant from the Virgin Mary, the wife of Joseph.
To instance only Matthew, since Faustus' argument refers to this Gospel, as the whole book cannot be quoted here, let whoever choose read it, and see how Matthew carries on to the passion and the resurrection the narrative of Him whom He calls the Son of David in the introduction to the genealogy. Of this same Son of David he speaks as being conceived and born of the Virgin Mary by the Holy Ghost. He also applies to this the declaration of the prophet, "Behold, a virgin shall conceive, and shall bear a son, and they shall call His name Emmanuel, which is being interpreted, God with us." Again, He who was called, even from the Virgin's womb, God-with-us, is said to have heard, when He was baptized by John, a voice from heaven, saying, "This is my beloved Son, in whom I am well pleased." Will Faustus say that to be called God is less than to be called the Son of God?
He seems to think so, for he tries to prove that because this voice came from heaven at the time of the baptism, therefore, according to Matthew, He must then have become the Son of God; whereas the same evangelist, in a previous passage, quotes the sacred announcement made by the prophet, in which the child born of the Virgin is called God-with-us.
Source: Reply to Faustus the Manichaean (New Advent)