God prepares the way for the promises of the covenant to be fulfilled. He does this by sending an angel to marry and asks her to carry the Messiah. Without a doubt, she said yes and Gods plan was on way. From this moment Mary was free from orginal sins and for her whole she was free of personal sin. God had Jesus born of virgin to show Jesus divine identity.
Though it wasn't just Mary who showed great faith, it was also Joesph. Though when Joesph first found he wanted to divorce her quietly. This would have caused harsh penalties to come upon Mary. Though God stepped in and had the angel appear to him in a dream and told him the same thing he told Mary. From that point on Joesph allowed Mary into his home.
Without God losing his divine nature he became man. For like the line, "for us man and our salvation he was incarnate of the virgin Mary and became." This phrase states that God is true God and true man. Though it sounds like a beautiful thing, it really wasn't. Jesus was born in a manger in Bethelhem.