There’s a whole scene relating to the birth of Jesus Christ that Luke paints for us, which we might tend to miss. To fully understand the Christmas story, we must juxtapose two stories: the birth of John and the birth of Jesus.
These two stories are placed side by side by the author, Luke, because to get the impact of Jesus in the manager, you’ve also got to get the impact of John in Elizabeth’s belly. In order to get the impact of Mary being a virgin, you have to also get the impact of Elizabeth being barren. In order to get the impact of Joseph being the espoused husband to Mary, you’re going to have to also get the impact of Zechariah going to the temple on his appointed day. In order to get the impact of Gabriel coming with the message to Mary, you must first see Gabriel being sent by God to Zechariah. In order to get the impact of the Jesus story, Luke, the investigative doctor, tells us we must also have the impact of the birth of John.
Luke tells us God created a situation that not only gave birth to John (whom the Old Testament predicted), but that God came up with a plan that miraculously produced John through Elizabeth and Zechariah. He made sure that Elizabeth and Zechariah were related to Mary because Mary would need Elizabeth and Zechariah to validate what was getting ready to happen to her. The birth of Jesus Christ, while miraculous in and of itself, is surrounded by an orchestration of the will of God.