The birth of Jesus was long awaited. The people of Israel had been expecting the Messiah for hundreds of years. They had been faithful to God some of the time, other times they took other directions. When the time had finally come for God to act, they were not in the least expecting it. The story of Advent can be seen as an example of how people respond when God does something unexpected.
The prophecy of Malachi was the last time God had spoken to his people, and when the narrative resumes in Luke, the echoes and references to Malachi shape the story as it continues into the fulfilment of the promise to send a redeemer.
Advent Sermon: God Keeps His Promises