Luke 1:26-38 (ESV)
In the sixth month the angel Gabriel was sent from God to a city of Galilee named Nazareth,  to a virgin betrothed to a man whose name was Joseph, of the house of David. And the virgin’s name was Mary.  And he came to her and said, “Greetings, O favored one, the Lord is with you!”  But she was greatly troubled at the saying, and tried to discern what sort of greeting this might be.  And the angel said to her, “Do not be afraid, Mary, for you have found favor with God.  And behold, you will conceive in your womb and bear a son, and you shall call his name Jesus.  He will be great and will be called the Son of the Most High. And the Lord God will give to him the throne of his father David,  and he will reign over the house of Jacob forever, and of his kingdom there will be no end.”
 And Mary said to the angel, “How will this be, since I am a virgin?”
 And the angel answered her, “The Holy Spirit will come upon you, and the power of the Most High will overshadow you; therefore the child to be born will be called holy— the Son of God.  And behold, your relative Elizabeth in her old age has also conceived a son, and this is the sixth month with her who was called barren.  For nothing will be impossible with God.”  And Mary said, “Behold, I am the servant of the Lord; let it be to me according to your word.” And the angel departed from her.
The angel Gabriel is sent to a virgin. That some people who believe in God can reject this account because of its impossibility is a very odd thing. If God can create the world He can cause a virgin to conceive. Belief in God necessitates belief in the miraculous. To be a Christian and not believe in miracles is an oxymoron in action. They are not nearly as prevalent as some people think (if miracles are common place, then they are no longer miracles) but God can violate the laws of nature if He chooses to. Did Jesus have to be born of a virgin? Probably not. But He was and it is important that He was. Why? Here are a few reasons, probably not all.
1) The virgin birth erases all doubt regarding the specialness of Jesus. Doubt could arise regarding whether He really was who He said He was. The virgin birth does not erase all doubts, but it does get a major one out of the way.
2) The virgin birth is God in action. Everything about our salvation is about God doing what we cannot and the way He was conceived is very much a part of it. Christians are chosen by God (Eph. 1:4). They are born again by God (John 3:3-8). They are given faith by God (Phil 1:29). They are gifted by God (I Cor. 12). They are kept from drifting away by God (I Peter 1:3-5). They are empowered by God (Eph. 6:10). They will be retrieved by God (I Thess. 4:13-18). The virgin birth is just God saying – “you can’t do this. I must do this for you.”
3) It is a beautiful combination of the human and the divine. Jesus would have been fully human and fully God regardless of how He cam into the world but this is a grander demonstration of both of His natures. He comes into the world in the usual way, by all appearances, and yet it was a miraculous birth of an eternal being who did not need normal biology to come to the planet.
There are no doubt more, but that will do for now. We believe in this event. It fuels the faith.