The gospels seemed to have been formed “backwards.” The initial preaching of the apostles was Christ Crucified and Risen. This is clear from Acts 2:23, 32, 3:14, 10:37-41, and 1 Cor 15:3-5. The teaching of Jesus (didache) was added to the “passion” of Jesus (kerygma). The last (canonical) stage of the development was to include a prologue concerning the origins of Jesus – was he simply a man? Matthew and Luke include miraculous birth stories, John has a theological prologue announcing that Jesus is the Word who was with God from the beginning since he is God. Notice the development taking mark as the earliest of the Gospels – there is no birth narrative and virtually nothing about his family history. The earlier one goes into the traditions of concerning Jesus, the less about Jesus’ birth we find.
Why were the infancy narratives written in the first place? Crossan thought the question should not be what Matthew and Luke tell us about the birth of Jesus, but “why they tell us anything at all?” What would motivate the gospel writer to include an explanation of the birth of Jesus? Raymond Brown suggested three reasons (Birth of the Messiah, 29).
The most simple explaination for the birth narratives is curiosity. Since Mark did not have many biographical details that people always seem to want to know about, the later gospels were interested in filling-in that gap.
Apologetic. One possible motivation for Luke’s presentation of John the Baptist’s birth along side Jesus’ birth is to show the superiority of Jesus over John, perhaps to answer non-Christian disciples of John (similar to those we meet in Acts 19) There is an apologetic value of the birth narrative when presenting the Gospel to skeptical Jews as well, helping to explain how the Messiah (who as to be born in Judean Bethlehem) ended up to be a native of Galilee. There is also the charge made by early Judaism that Jesus as of illegitimate birth, answer by both evangelists by the explanation of a virgin birth.
There are obvious theological motives as well. The genealogy in Matthew connects Jesus to David, Moses, Joseph, and the other great men in the history of Israel. Like Moses he survives the slaughter of children by a pagan ruler, and like Moses he goes to the mountain to dispense the Law (Matthew 5-7). There is a developing Christology in the four Gospels, Mark tells us that Jesus is already the Son of God at the baptism. In the next two gospels (Matthew and Luke are chronological about the same time), Jesus is God from the moment of his conception, and in John he is God from the very beginning. In fact, John tells us Jesus is equal with God from eternity since he is the creator (John 1:1).
I would add a fourth motivation for Matthew and Luke including the birth narratives. More than Mark, these two gospels are interested in showing that Jesus fulfilled prophecy, beginning with his birth. Readers familiar with the Old Testament know than God has done a number of miracles to bring special individuals into the world – Isaac and Samuel are examples of children born to elderly or barren parents. Jesus is the ultimate “miracle child” since he was born from a virgin.
All of this highlights the uniqueness of Jesus at the very beginning of the story. What might be a few other motives for the writers of the gospels to include the story of Jesus’ birth? Or to think of it the other way, why did Mark and John omit the brith of Jesus?