Mary’s untimely premature pregnancy was bad enough. That Joseph, her fiance was about to divorce her would have perhaps made the situation unbearable for Mary. So, God sends an “angel.”
Matthew 1:20 reads: But after he had considered this, an angel of the Lord appeared to him in a dream and said, ?Joseph son of David, do not be afraid to take Mary home as your wife, because what is conceived in her is from the Holy Spirit.”
Joseph was doing what he believed was right. Though the Torah does not say a man must divorce his wife if he discovers her pregnant before marriage, but it gets very close to this. (Read Deut 22.) One can only imagine Joseph arguing with God — I’m doing this because I’m a righteous man (a tsadiq), I’m doing this because of Your Word, Your Torah! And God is telling him that this is God’s plan, a new plan, etc. etc.
But for Joseph to be convinced, God sends an angel and the word “angel” figures prominently in Matthew’s account of Christmas.
Angels are messengers from God. They are Prophets on Steroids, as it were. If the prophet listens to God and speaks for God, Angels do the same only their messages are the kinds that require special communication because the messages are too incredible to believe. So, God sends an angel.
Christmas rests on this angel’s message and Joseph’s response. “It’s OK, Joseph, for you to do what you think is wrong. Mary’s pregnancy is not what you think. This is an act of God, a supernatural pregnancy. Marry Mary.”
Joseph does. Why? Because apart from the Angel Joseph would not have believed the words he was hearing. He was too committed to the Torah to think God would even imagine something like this.