13 For this cause too the angel came bringing His name from Heaven, hereby again intimating that this is a wondrous birth: it being God Himself who sends the name from above by the angel to Joseph. For neither was this without an object, but a treasure of ten thousand blessings. Wherefore the angel also interprets it, and suggests good hopes, in this way again leading him to belief. For to these things we are wont to be more inclined, and therefore are also fonder of believing them.
So having established his faith by all, by the past things, by the future, by the present, by the honor given to himself, he brings in the prophet also in good time, to give his suffrage in support of all these. But before introducing him, he proclaims beforehand the good things which were to befall the world through Him. And what are these? Sins removed and done away. “For He shall save His people from their sins.”
Here again the thing is signified to be beyond all expectation. For not from visible wars, neither from barbarians, but what was far greater than these, from sins, he declares the glad tidings of deliverance; a work which had never been possible to any one before.
But wherefore, one may ask, did he say, “His people,” and not add the Gentiles also? That he might not startle the hearer yet a while. For to him that listens with understanding he darkly signified the Gentiles too. For “His people” are not the Jews only, but also all that draw near and receive the knowledge that is from Him.
And mark how he has by the way discovered to us also His dignity, by calling the Jewish nation “His people.” For this is the word of one implying nought else, but that He who is born is God's child, and that the King of those on high is the subject of his discourse. As neither does forgiving sins belong to any other power, but only to that single essence.
Source: Homilies on the Gospel of St. Matthew (New Advent)