Jewish betrothal was different kind of engagement from what the West experiences. It was not an easily broken relationship, but a real contract. It was initiated by the couple's parents, likely when the girl was anywhere from 12-14 years old and the man 18-24 years old. It was a legal arrangement, witnessed by two other people. Once entered into, it had the legal force of a marriage. If the man died during the year prior to the consummation, the woman was considered a widow. If she was intimate with another man, she was guilty of adultery. Mary's situation meant she was guilty of adultery.
Joseph did not plan for this. He found himself making contingencies. Legally, he was entitled to divorce her, and if he wanted, in a public manner. But Matthew 1:19 says he was a "righteous man" (NASB) and decided to do it quietly.
God intervened, sending a dream to Joseph: the voice of an angel.
"Son of David, do not be afraid to take Mary as your wife; for the child who has been conceived in her is the Holy Spirit. She will bear a son; and you shall call his name Jesus, for he will save his people from their sins." (1:20)This is an incredible message: this child is from the Holy Spirit. Not only that, but he's going to save people from their sins. That is quite a pedigree. The angel also tells Joseph that he will name the child Jesus. Joseph, in giving the son his name admits his paternity. Joseph will be Jesus' earthly father.
Furthermore, the angel tells him:
Now all this took place to fulfill what was spoken by the Lord through the prophet: "Behold the Virgin shall conceive and shall bear a son, and they shall call him Immanuel, which translated means 'God with us.'"Joseph had a plan, but God had another plan; one that was established long before Joseph was alive. It was a plan foretold by the prophets. Joseph had no idea that he was destined to be part of a such a miraculous event. He thought he was just getting engaged to a girl.
Theologically, this account in Matthew is important for its Christological teaching. Jesus is a child of the Holy Spirit and the child of Mary: divine and human. It also demonstrates the sovereign control of God over his purposes. There was no way Joseph was going to foil the plans of God. That God's plans are certain can and should be a comfort to us, even when those plans are difficult.
Practically speaking, we are reminded that we, like Joseph, are part of God's sovereign working. Most of us will not be in the midst of something as fantastic as the Incarnation of Christ, but we each live in the tension between the plans we devise for ourselves and the plans God has for us. Plans we believe are of our own making often turn out to have nothing to do with us and everything to do with God's plans. And God has ways of thwarting attempts to take matters into our own hands when we don't like the plans he has for us.
I love that these truths occur in a passage which draws our thoughts to Christmas. If there was ever a time for planning and schedules and deadlines, it's Christmas in North America. We place a lot of importance on each detail of our plan: the perfect decorations, the food, the perfect gift. But if God intervenes with another plan, we will not be able to stop them. Last year, my friend lost her mother two days before Christmas. How's that for a change in plans? This year, more than one person will have the same change in plans. We best not hold our plans too closely, because over it all, God may have another plan. And in the end, however difficult it may be, God's plans are always good.