It's a funny thing about the Bible and local congregations. Much of it is totally opaque to the average church goer, and this includes passages that are so familiar that worshippers more or less "shut down" while they are being read or preached upon. The Christmas stories ("birth narratives" in scholar-speak) are just such passages. This includes Matthew 1:18-25, which is the story of how Joseph decided to stick with Mary even though she was pregnant by the Holy Spirit. It's a familiar story, which after countless Advent re-readings is just not listened to by the congregation.
It is ironic, then, that this unsurprising, familiar passage is actually filled with surprises. For ancient audiences, that may well have been one of the key components of the story. As the story goes, Mary was mysteriously pregnant (surprise!), and Joseph decided to handle this unhappy surprise judiciously. But, then an angel shows up in a dream (surprise!) and tells him to go ahead and marry Mary. The angel also reveals something of the nature of the child. So, Joseph's marital plans change again and he ends up being married (surprise!) when he thought that he'd lost his prospective bride to an unwanted premarital pregnancy. Surprise, surprise, and surprise. Interestingly, the theme of surprise is carried on in the next story in Matthew (Matthew 2:1-12) when the three gentlemen "from the east" appeared suddenly in Jerusalem seeking the newly born Messiah (surprise!). It is clear that King Herod and the people of Jerusalem were hugely surprised by their appearance and inquiries.
One point we can take away from the prominent role of surprise in the opening chapters of Matthew is that faith and spirituality are not really about certainty and getting control of one's life. In the Spirit, there is always an element of the unexpected. Matthew's readers, thus, are warned that if they buy into the Jesus Movement they can expect the unexpected to happen to them. They can expect to be surprised by the Spirit.