And she shall bring forth a son, and you shall call his name JESUS: for he shall save his people from their sins.
All Commentaries on Matthew 1:21 Go To Matthew 1
Theophylact of Ochrid
So that no one could ask, "How can I believe you that she has conceived of the Holy Spirit?" the angel speaks of the future, saying that she shall bear a son. "For if I tell the truth in this matter, it is clear that what I said concerning her conception of the Holy Spirit is also true." The angel did not say, "She shall bear you a son," but simply, "She shall bear a son." For Mary did not bring forth for Joseph’s sake, but for the whole world; nor did this grace concern him alone, but it was poured out on all.
"Thou shalt call," as though you were His father, and as the protector of the Virgin. For you must not think, Joseph, that because the conception is of the Holy Spirit that you can leave the Virgin helpless, but rather you will serve her in all things.
Here he interprets the name "Jesus," showing that it means "Saviour." The angel says that Jesus will save His people, not only the Jewish people, but also the Gentiles who are eager to believe and to become His people. From whom will He save them? Perhaps from enemies? No, but from their sins. Hence it is clear that it is God Who will be born, for it is the attribute of God alone to forgive sins.