Isaias 7:13

3. For as He became man in order to undergo temptation, so also was He the Word that He might be glorified; the Word remaining quiescent, that He might be capable of being tempted, dishonoured, crucified, and of suffering death, but the human nature being swallowed up in it (the divine), when it conquered, and endured [without yielding], and performed acts of kindness, and rose again, and was received up [into heaven]. He therefore, the Son of God, our Lord, being the Word of the Father, and the Son of man, since He had a generation as to His human nature from Mary, who was descended from mankind, and who was herself a human being, was made the Son of man. …

4. For the one and the same Spirit of God, who proclaimed by the prophets what and of what sort the advent of the Lord should be, did by these elders give a just interpretation of what had been truly prophesied; and He did Himself, by the apostles, announce that the fulness of the times of the adoption had arrived, that the kingdom of heaven had drawn nigh, and that He was dwelling within those that believe on Him who was born Emmanuel of the Virgin. …

5. And when He says, “Hear, O house of David,” He performed the part of one indicating that He whom God promised David that He would raise up from the fruit of his belly ( ventris ) an eternal King, is the same who was born of the Virgin, herself of the lineage of David. For on this account also, He promised that the King should be “of the fruit of his belly ,” which was the appropriate [term to use with respect] to a virgin conceiving, and not “of the fruit of his loins ,” nor “of the fruit of his reins ,” which expression is appropriate to a generating man, and a woman conceiving by a man. In this promise, therefore, the Scripture excluded all virile influence; yet it certainly is not mentioned that He who was born was not from the will of man. …

Begin we, therefore, to prove that the Birth of Christ was announced by prophets; as Isaiah ( e.g. ,) foretells, “Hear ye, house of David; no petty contest have ye with men, since God is proposing a struggle. Therefore God Himself will give you a sign; Behold, the virgin shall conceive, and bear a son, and ye shall call his name Emmanuel” (which is, interpreted, “God with us”): “butter and honey shall he eat;”: “since, ere the child learn to call father or mother, he shall receive the power of Damascus and the spoils of Samaria, in opposition to the king of the Assyrians.”

The same rule of truth teaches us to believe, after the Father, also on the Son of God, Christ Jesus, the Lord our God, but the Son of God, of that God who is both one and alone, to wit the Founder of all things, as already has been expressed above. For this Jesus Christ, I will once more say, the Son of this God, we read of as having been promised in the Old Testament, and we observe to be manifested in the New, fulfilling the shadows and figures of all the sacraments, with the presence of the truth embodied. For as well the ancient prophecies as the Gospels testify Him to be the son of Abraham and the son of David. …

“And when he says ‘Hear ye now, Oh House of David’ he means that the everlasting King whom God promised to David that he would raise up from his body is He who was born of David’s Virgin.”