"How can this be?"
When the first Christmas happened most of the world was unaware of it. There were many signs of God's activity but they could have been dismissed as coincidence, exaggeration or rumor. The book of Esther is the only book in the Bible that does not mention God. Yet God is obviously active in it all. Nine times we are told that King Xerxes sent out a decree(2:8,3:15, 4:3,8, 8:14,17, 9:1,13,14). Each time the king thought he was in control, but the LORD was working out His plan in it all. Some of the King's decrees were motivated by evil and some by good, but the LORD was working out His plan. Ultimately, God's plan for us is peace and security. Years later another decree went out from another King who thought he was in charge. Caesar's decree to tax the world led to a birth and an announcement of peace on earth.
Even when we cannot see Him and sometimes when we don't even care, God is at work. He wants us to have peace and security. The Devil will try to cause events that damage and detour our lives, but the Lord God Almighty is on our side. Christmas reminds us that God is at work in His world. A virgin becoming pregnant is an amazing thing! It was no exaggeration or rumor. It was God at work--because that is what God does. How can this be? He is. And He is God!
"Then let us all with one accord sing praises to our heavenly Lord. Who hath made heav'n and earth of naught, and with His blood mankind hath bought."