2 And again, speaking in reference to the angel, he says: “But at that time the angel Gabriel was sent from God, who did also say to the virgin, Fear not, Mary; for you have found favour with God.” And he says concerning the Lord: “He shall be great, and shall be called the Son of the Highest: and the Lord God shall give unto Him the throne of His father David: and He shall reign over the house of Jacob for ever; and of His kingdom there shall be no end.” For who else is there who can reign uninterruptedly over the house of Jacob for ever, except Jesus Christ our Lord, the Son of the Most High God, who promised by the law and the prophets that He would make His salvation visible to all flesh; so that He would become the Son of man for this purpose, that man also might become the son of God?
And Mary, exulting because of this, cried out, prophesying on behalf of the Church, “My soul does magnify the Lord, and my spirit has rejoiced in God my Saviour. For He has taken up His child Israel, in remembrance of His mercy, as He spoke to our fathers, Abraham, and his seed for ever.” By these and such like [passages] the Gospel points out that it was God who spoke to the fathers; that it was He who, by Moses, instituted the legal dispensation, by which giving of the law we know that He spoke to the fathers.
This same God, after His great goodness, poured His compassion upon us, through which compassion “the Day-spring from on high has looked upon us, and appeared to those who sat in darkness and the shadow of death, and has guided our feet into the way of peace;” as Zacharias also, recovering from the state of dumbness which he had suffered on account of unbelief, having been filled with a new spirit, did bless God in a new manner. For all things had entered upon a new phase, the Word arranging after a new manner the advent in the flesh, that He might win back to God that human nature (hominem) which had departed from God; and therefore men were taught to worship God after a new fashion, but not another god, because in truth there is but “one God, who justifies the circumcision by faith, and the uncircumcision through faith.” But Zacharias prophesying, exclaimed, “Blessed be the Lord God of Israel; for He has visited and redeemed His people, and has raised up an horn of salvation for us in the house of His servant David; as He spoke by the mouth of His holy prophets, which have been since the world begun; salvation from our enemies, and from the hand of all that hate us; to perform the mercy [promised] to our fathers, and to remember His holy covenant, the oath which He swore to our father Abraham, that He would grant unto us, that we, being delivered out of the hand of our enemies, might serve Him without fear, in holiness and righteousness before Him, all our days.” Then he says to John: “And you, child, shall be called the prophet of the Highest: for you shall go before the face of the Lord to prepare His ways; to give knowledge of salvation to His people, for the remission of their sins.” For this is the knowledge of salvation which was wanting to them, that of the Son of God, which John made known, saying, “Behold the Lamb of God, who takes away the sin of the world. This is He of whom I said, After me comes a man who was made before me; because He was prior to me: and of His fullness have all we received.” This, therefore, was the knowledge of salvation; but [it did not consist in] another God, nor another Father, nor Bythus, nor the Pleroma of thirty Æons, nor the Mother of the (lower) Ogdoad: but the knowledge of salvation was the knowledge of the Son of God, who is both called and actually is, salvation, and Saviour, and salutary. Salvation, indeed, as follows: “I have waited for Your salvation, O Lord.” And then again, Saviour: “Behold my God, my Saviour, I will put my trust in Him.” But as bringing salvation, thus: “God has made known His salvation (salutare) in the sight of the heathen.” For He is indeed Saviour, as being the Son and Word of God; but salutary, since [He is] Spirit; for he says: “The Spirit of our countenance, Christ the Lord.” But salvation, as being flesh: for “the Word was made flesh, and dwelt among us.” This knowledge of salvation, therefore, John did impart to those repenting, and believing in the Lamb of God, who takes away the sin of the world.
Source: Adversus haereses (New Advent)