"The virgin will be with child and will give birth to a son, and they will call him Immanuel" which means, "God with us." NIV
The angel came to Mary bearing this wonderful news. It was a fulfillment of God's promise and covenant to Abraham that His seed would become a great nation, He would bless him, make his name great and that he would be a blessing to others; which is implying the coming Messiah through Abraham's descendants.
As we look upon this Christmas season and give thanks for Jesus birth let us also give thanks to God for His faithfulness to all men that He keeps His promises that Christ who dwells in our hearts by faith to those who believe, is the complete fulfillment of that promise.
Before Jesus was crucified He told his disciples that it were better that He go away that the comforter could come. He was referring to the Holy Spirit being poured out. He said this because Jesus knew that it was better that He go away and no longer live "among" them, so that He could return through the Holy Spirit to live "within" them. "Immanuel" which means "God with us!" Christ in us, the hope of Glory.