The interest in the new movie, the Nativity Story (see trailer below), has stirred a renewed interest in the individuals that surrounded the birth of Jesus, and especially Mary, His mother.
Why was it necessary for Mary to be a virgin? This, it turns out, is more than just an interesting or incidental fact about Jesus' birth. It was absolutely essential that the Messiah arrive on our planet through a divine conception. It boils down to this: Jesus needed to be both fully God and fully human.
The Bible is a bit vague on the details, but it describes the conception of Jesus like this. A angelic messenger from God told Mary, "The Holy Spirit will come upon you, and the power of the Most High will overshadow you. So the holy one to be born will be called the Son of God." (Luke 1:35). While we are not privy to the genetic specifics, it is clear that Jesus received his human nature from his mother. Because of this, he was fully human. He felt like us, lived like us, and bled like us. It is imprecise to say that, at conception, Jesus received his divine natue from God, since Jesus already existed before that. It is more accurate to say that somehow when the power of the Most High overshadowed Mary that Jesus, while never shedding his divinity, also embraced a human body. And while Jesus left behind some of the attributes of diety (for example, while on Earth he was no longer omnipresent -- in every place at the same time), he never lost his God-nature. Jesus was both fully God and fully human. As a figure of speech, we might say that Jesus had both human and divine DNA.
Therefore, because he was God, Jesus could bring God to us.
Because he was human, Jesus could bring us to God.
Because he was God, Jesus could take upon himself the sins of the world.
Because he was human, Jesus could identify with our humanity, weakness and temptation.
Because he was God, Jesus can stand as our advocate in Heaven.
Because he is human we can be assured that the God of Heaven knows how we feel.