How?

The angel went to her and said, “Greetings, you who are highly favored! The Lord is with you.”

Mary was greatly troubled at his words and wondered what kind of greeting this might be. But the angel said to her, “Do not be afraid, Mary; you have found favor with God. You will conceive and give birth to a son, and you are to call him Jesus.”

“How will this be,” Mary asked the angel, “since I am a virgin?”

The angel answered, “The Holy Spirit will come on 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 1:28-32, 34-35 New International Version (NIV)

Is it possible to truly understand the birth of Jesus Christ? Probably not any more than we really understand how God created us. We have explanations of both in God's Word. Absent any other absolute power of reasoning, we must simply believe. And that is just what God has asked us to do. Mary believed and therefore received. Her faith brought her favor and brought us a Savior.

God is with us.

Therefore the Lord himself will give you a sign: The virgin will conceive and give birth to a son, and will call him Immanuel. Isaiah 7:14 NIV