Not to hijack - but I think this point is related. Why are the nativity narratives so different?
Matthew wrote for a largely Jewish audience and Luke for a largely gentile one.
Matthew:
What is Matthew telling his audience when he includes in his infancy narrative the accounts of the star, the magi, Herod’s plot, the massacre of the babies, and the flight into Egypt? Each of these images recalls to the mind of Matthew’s largely Jewish audience something significant in their past, something that touches their minds and hearts because it reminds them of their history, their hopes, and their sense of purpose and destiny.
We must remember that when Matthew was arranging his inherited oral and written traditions into the Gospel that we now have, he was ever mindful of the needs of his Jewish audience. He was writing to people of the covenant who wanted to remain faithful to that covenant. Matthew presents Jesus to his audience as the new Moses who has authority from God to give the new law. This basic concern is reflected in both the form and the content of Matthew’s Gospel. He organizes his Gospel into five sections that reflect the Pentateuch, the five books of the old law. He pictures Jesus giving the new law on a mountain, just as Moses did. Everywhere in Matthew’s Gospel we read of Jesus against the backdrop of the history of the Jewish people, for it is only against this backdrop that Jesus’ true identity can be understood by Matthew’s largely Jewish audience.
Our goal, then, as we read Matthew’s infancy narrative, is to allow his use of Old Testament images to work, letting the imagery recall to our minds those same images that Matthew intended to recall in the minds of his audience.
Luke:
The images that Luke weaves into his account of Jesus’ birth are very different than those used by Matthew. Still, the intention behind the use of images is the same. By incorporating these images Luke is helping his audience understand the significance of the event of Jesus’ birth, a significance that was understood only in the light of the resurrection.
To understand the depths of Luke’s message we must understand the meanings behind the images of the firstborn son, wrapped in swaddling clothes and lying in a manger. When these images are explained, the appropriateness of the announcement to the shepherds will be evident. Mary gave birth to *her firstborn *(Luke 2:7). What would the word *firstborn *recall for Luke’s audience? By the time Luke is collecting material for his Gospel, around A.D. 85, the term *firstborn *had become a reference to Jesus’ divinity. To understand the significance of the word to Luke’s Gentile audience we need to know a little about the way an understanding of Jesus’ divinity had developed and had been expressed in the early church.
When Jesus was alive on earth his followers did not recognize him. They were like the people of Isaiah’s time. They too did not recognize their God. Isaiah begins his prophecy by picturing God lamenting this fact.
However, after the resurrection, and by the time the infancy narratives were taking form, Jesus’ followers did recognize their Lord, The leaders of the church, the shepherds, did recognize the food for their flock. This is why Luke pictures the angels announcing the birth of the child to the shepherds and the shepherds believing and rushing to Bethlehem to see the baby lying in the manger. Now, Israel does recognize her Lord.
Luke’s apparently simple imagery of the babe in the manger greeted by shepherds is Luke’s way of announcing to his Gentile audience that the baby Jesus, the risen Lord, is now recognized as both God and man, the source of life for his people.
(For a thorough scholarly study of the infancy narratives in Matthew and Luke see Raymond E. Brown, The Birth of the Messiah, Garden City, N.Y.: Image Books, 1977.)