The angel answered her, "The Holy Spirit will come on you, and the power of the Most High will overshadow you. Therefore also the holy one who is born from you will be called the Son of God.
And, of course, throughout the Old Testament prophesies, there was that promise that the Messiah would sit upon the throne of David, to order it, and to establish it in righteousness and in judgment, from henceforth, even forever.