2 That John knew the one and the same Word of God, and that He was the only begotten, and that He became incarnate for our salvation, Jesus Christ our Lord, I have sufficiently proved from the word of John himself. And Matthew, too, recognising one and the same Jesus Christ, exhibiting his generation as a man from the Virgin, even as God did promise David that He would raise up from the fruit of his body an eternal King, having made the same promise to Abraham a long time previously, says: “The book of the generation of Jesus Christ, the son of David, the son of Abraham.” Then, that he might free our mind from suspicion regarding Joseph, he says: “But the birth of Christ was on this wise.
When His mother was espoused to Joseph, before they came together, she was found with child of the Holy Ghost.” Then, when Joseph had it in contemplation to put Mary away, since she proved with child, [Matthew tells us of] the angel of God standing by him, and saying: “Fear not to take unto you Mary your wife: for that which is conceived in her is of the Holy Ghost. And she shall bring forth a son, and you shall call His name Jesus; for He shall save His people from their sins.
Now this was done, that it might be fulfilled which was spoken of the Lord by the prophet: Behold, a virgin shall conceive, and bring forth a son, and they shall call His name Emmanuel, which is, God with us;” clearly signifying that both the promise made to the fathers had been accomplished, that the Son of God was born of a virgin, and that He Himself was Christ the Saviour whom the prophets had foretold; not, as these men assert, that Jesus was He who was born of Mary, but that Christ was He who descended from above.
Matthew might certainly have said, “Now the birth of Jesus was on this wise;” but the Holy Ghost, foreseeing the corrupters [of the truth], and guarding by anticipation against their deceit, says by Matthew, “But the birth of Christ was on this wise;” and that He is Emmanuel, lest perchance we might consider Him as a mere man: for “not by the will of the flesh nor by the will of man, but by the will of God was the Word made flesh;” and that we should not imagine that Jesus was one, and Christ another, but should know them to be one and the same.
Source: Adversus haereses (New Advent)