2 I will, for the present, suppose that this person was right in saying that the son of David was born of Mary. It still remains true, that in this whole passage of the generation no mention is made of the Son of God till we come to the baptism; so that it is an injurious misrepresentation on your part to speak of this writer as making the Son of God the inmate of a womb. The writer, indeed, seems to cry out against such an idea, and in the very title of his book to clear himself of such blasphemy, asserting that the person whose birth he describes is the son of David, not the Son of God.
And if you attend to the writer's meaning and purpose, you will see that what he wishes us to believe of Jesus the Son of God is not so much that He was born of Mary, as that He became the Son of God by baptism at the river Jordan. He tells us that the person of whom he spoke at the outset as the son of David was baptized by John, and became the Son of God on this particular occasion, when about thirty years old, according to Luke, when also the voice was heard saying to Him, "You are my Son; this day have I begotten You." It appears from this, that what was born, as is supposed, of Mary thirty years before, was not the Son of God, but what was afterwards made so by baptism at Jordan, that is, the new man, the same as in us when we were converted from Gentile error, and believe in God.
This doctrine may or may not agree with what you call the Catholic faith; at all events, it is what Matthew says, if Matthew is the real author. The words, You are my Son, this day I have begotten You, or, This is my beloved Son, in whom I am well pleased, do not occur in connection with the story of Mary's motherhood, but with the putting away of sin at Jordan. This is what is written; and if you believe this doctrine, you must be called a Matthæan, for you will no longer be a Catholic.
The Catholic doctrine is well known; and it is as unlike Matthew's representations as it is unlike the truth. In the words of your creed, you declare that you believe in Jesus Christ, the Son of God, who was born of the Virgin Mary. According to you, therefore, the Son of God comes from Mary; according to Matthew, from the Jordan; while we believe Him to come from God. Thus the doctrine of Matthew, if we are right in assigning the authorship to him, is as different from yours as from ours; only we acknowledge that he is more cautious than you in ascribing the being born of a woman to the son of David, and not to the Son of God.
As for you, your only alternative is to deny that those statements were made, as they appear to be, by Matthew, or to allow that you have abandoned the faith of the apostles.
Source: Reply to Faustus the Manichaean (New Advent)