22 All this has taken place so that there might be fulfilled what was spoken by the Lord through the prophet, saying,
23 ‘Behold, the young woman will conceive and will bear a son, and they will call his name his name Immanuel’, which is translated ‘God with us’.
The announcement that Jesus will ‘save his people from their sins’ is followed immediately by the reference to the prophecy in Isaiah about a virgin or young woman who will bear a son whose name will be Immanuel (Isaiah 7:14).
This child will not actually do anything: he does not grow up to become a saviour or king. But his birth will be a sign to king Ahaz that within a few years Judah will be overrun by the Assyrians because he was unwilling to trust God (7:10-13): ‘The Lord will bring upon you and upon your people and upon your father’s house such days as have not come since the day that Ephraim departed from Judah – the king of Assyria’ (7:17). The river of the Assyrian army will flow into the land of the child Immanuel (8:8), but it will not in the end sweep away Jerusalem and the house of David. The fact that ‘God is with us’ is an assurance that when Jerusalem faces military disaster, God will not allow his people to be completely overwhelmed (8:9-10).
Matthew transfers this paradigm to the situation faced by first century Israel: the birth of Jesus is similarly a sign - by virtue of its exceptional circumstances - that when the nation faces destruction, God will nevertheless be present and will preserve those who fear him. This is what it means to ‘save his people from their sins’ (Matt. 1:21). For Matthew the birth of Jesus is eschatologically significant - it has to do with the end of the age of second temple Judaism. It is not made an argument for an incarnational understanding of the relationship between Jesus and God.