If you really want to understand the birth of Jesus you have to look back to the Old Testament. One reason I can say that with confidence is that all four Gospels start by pointing backward. Matthew starts with a genealogy going back to David and Abraham. The NIV uses the word "genealogy" to translate the Greek word "Genesis" which means "beginning." Mark skips Jesus birth but does just the word beginning (arche, not genesis) to start wit the beginning of Jesus' ministry. Luke starts with the birth of John the Baptist and also, like Matthew, also gives us a genealogy going back to Adam. John starts with a phrase taken from Genesis 1, "In the beginning" (like Mark, the word arche). All four Gospels start with the idea of beginning and three of the four (the three that have anything to say about the Incarnation) have a beginning that is not the birth of Jesus. The all point backward to the Old Testament. Even Mark starts by pointing back to the Old Testament as he quotes Isaiah only 2 verses into his gospel and then has John the Baptist reference Jesus as the "Lamb of God."The birth of Jesus is not the start of the Gospel. The Gospel writers knew that if you want to know the full story of Jesus you have to look back to the Old Testament. So let's do that for a minute. Let's start with Matthew 1:1,You know more Greek than you thought...“Biblos geneseos Isou Christou”Literally – [This is the] book of [the] Genesis of Jesus Anointed.Matthew tells us exactly who Jesus is. He is the Messiah (Hebrew word for Anointed. This is Christ in Greek). He then backs up that assertion with his evidence, the family tree of Jesus. This family tree gives us both the expected and the unexpected origins of Jesus. First the expected - his lineage through Abraham shows he is Jewish and a member of the covenant and recipient of the covenant promises and blessings of Abraham. Then comes his connection with David, making him in line for the divine promise of a ruler to come from the lineage of David. This means Jesus is eligible to be the Messiah. He doesn't violate the family tree criteria.But there is more...the unexpected people in the family tree:

• Tamar (1:3) - She was a Canaanite who had a child through her father-in-law Judah. The levirate marriage laws required the brother of her deceased husband to bear children for his brother in his deceased brother's name. Her deceased husband's brother refused and so she was determined to have a child out of her husband's blood line. So she tricked/seduced her father-in-law Judah into impregnating her. This child is in the lineage of Jesus.
• Rahab (1:5) - Another Canaanite who didn't play the part of a prostitute to be impregnated by her father-in-law. She was a prostitute. She was spared in Jericho and ended up in the family tree of Jesus.
• Ruth (1:5) - A Moabite who married Boaz. It is possible that was also another questionable sexual exploit like Tamar and Rahab but that is in dispute. She was, like Tamar and Rahab, a foreigner to Israel.
• Bathsheba (1:6) - She was married to Uriah the Hittite (a foreigner) and had an affair with David.
• Mary (1:16) - poor, teenage pregnant mom who had baby Jesus out of wedlock in a backroom stable at an out of town house. Obviously, that wasn't anything scandalous when the truth is told and understood as she was with child via the Holy Spirit and not by her own sinful behavior.

Then come the shepherds. We read that as a quaint detail. Shepherds were typically at the bottom of the social scale. They were typically pretty poor. It says a lot about a God who would first reveal the unfolding mystery of the Gospel that the angels and prophets longed to know to shepherds. Joel Green, in his fantastic commentary on Luke, points out that this is a direct connection with Mary's song in Luke 1, particularly 1:52, "He has brought down rulers from their thrones but has lifted up the humble." We see the unexpected come out on top over the expected.These are the unexpected ones. It is no wonder that Jesus had a heart for the tax collectors and "sinners" and that he allowed table fellowship with people of questionable character and sexual history. Jesus was without sin and yet he could relate to these people because these were his people...sheep without a shepherd. So we start with the family tree of Jesus because it helps make sense out of not even just the birth of Jesus but also the life and ministry of Jesus. We are reminded that even though we sin, Jesus still has a place for us.In Luke 14:15-24 he tells a parable about a wedding banquet. The first people invited were those one would expect to attend a celebration of this magnitude but they refuse to come. One just bought a field. Another just bought some oxen he wants to test drive. Still another just got married. Many commentators have noticed that these three things were in the Torah regarding those excluded from having to go to war in this exact order (property, work and marriage - Deut 20:5-9). Many have also noticed that those who were invited in the remaining verses of the parable were the "unclean" ones of society (see below). Jesus is reversing the expectation in his new way of doing things...in his own "holy war" so to speak (see Swartley's Israel's Scripture Traditions in the Synoptic Gospels, 136-137). Jesus is fighting a different kind of fight and he invites us to a table of the unexpecteds. This was nothing new...the baby in the manger warned us this was coming!Here is how the parable ends,“The servant came back and reported this to his master. Then the owner of the house became angry and ordered his servant, ‘Go out quickly into the streets and alleys of the town and bring in the poor, the crippled, the blind and the lame.’“‘Sir,’ the servant said, ‘what you ordered has been done, but there is still room.’Then the master told his servant, ‘Go out to the roads and country lanes and compel them to come in, so that my house will be full. I tell you, not one of those who were invited will get a taste of my banquet.’”The margins are exactly where Jesus finds those willing to accept his invitation. It is entirely possible to think you are on the guest list but not even know that you received an invitation. The invitation might come from the mentally challenged person who "interrupts" your Bible class, the child who wants your attention so badly that they misbehave to get it or the person you hesitate to make eye contact with because you know they will ask you for money. These are the daily invitations we receive...invitations from the margins...invitations from the least of these to participate in something that we might not choose otherwise but are at the very heart and soul of what the Gospel is all about.