2 But what are the things set before us today? “Now all this was done, that it might be fulfilled which was spoken of the Lord by the prophet, saying.” In a tone worthy of the wonder, with all his might he has uttered his voice, saying, “Now all this was done.” For when he saw the sea and the abyss of the love of God towards man, and that actually come to pass which never had been looked for, and nature's laws broken, and reconciliations made, Him who is above all come down to him that is lower than all, and “the middle walls of partition broken,” and the impediments removed, and many more things than these done besides; in one word he has put before us the miracle, saying, “Now all this was done that it might be fulfilled which was spoken of the Lord.” For, “think not,” says he, “that these things are now determined upon; they were prefigured of old.” Which same thing, Paul also everywhere labors to prove.
And the angel proceeds to refer Joseph to Isaiah; in order that even if he should, when awakened, forget his own words, as newly spoken, he might by being reminded of those of the prophet, with which he had been nourished up continually, retain likewise the substance of what he had said. And to the woman he mentioned none of these things, as being a damsel and unskilled in them, but to the husband, as being a righteous man and one who studied the prophets, from them he reasons. And before this he says, “Mary, your wife;” but now, when he has brought the prophet before him, he then trusts him with the name of virginity; for Joseph would not have continued thus unshaken, when he heard from him of a virgin, unless he had first heard it also from Isaiah. For indeed it was nothing novel that he was to hear out of the prophets, but what was familiar to him, and had been for a long time the subject of his meditations. For this cause the angel, to make what he said easy to be received, brings in Isaiah. And neither here does he stop, but connects the discourse with God. For he does not call the saying Isaiah's, but that of the God of all things. For this cause he said not, “that it might be fulfilled which was spoken of Isaiah,” but “which was spoken of the Lord.” For the mouth indeed was Isaiah's, but the oracle was wafted from above.
Source: Homilies on the Gospel of St. Matthew (New Advent)