The angel comes to Joseph in a dream. The angel not only assures Joseph that he should marry Mary, but also explains what the birth of the child means.
Joseph is to call the child Jesus. In Hebrew this is Joshua. Its meaning is connected to salvation. Hence the explanation (1.21) that he would save his people from their sins. It is important to notice that he is not said to save them from their political enemies. This is what leaders in the Old Testament did. This saviour will deal with the problem that is in everyone’s heart.
The angel says that the child will be called Immanuel. This seems to be a description rather than a name. It tells us a very important thing about the child. He is God. God with us. The connection with the prophecy of Isaiah is clear. The idea that God would come to be with his people is very common in the prophets. Especially that he would come to save them.
The angel explains how Mary has become pregnant. It is by the Holy Spirit (1.20). This is another way of understanding how the child can be both God with us and also Mary’s child.
The rest of Matthew’s gospel will outline what it means for Jesus to be God with his people and how he saves them from their sins.
Dale