It is instructive to compare the reading from St. Paul and the reading from St. Matthew.  St. Paul in Romans tersely describes the coming of Jesus: "...his Son, descended from David according to the flesh".  As for today's Gospel, St. Matthew tells gives us an annuciation story.  The essential element of this brief narrative is the announcement that Mary has conceived a son and that this son is to be called Jesus because (as the name signifies in Hebrew) he will save.  Matthew adds two elements on his own: first, that Jesus merits the name Immanuel (which means 'with us is God') and second, that the child was born of a virgin because there was no intercourse between Joseph and Mary before his birth.  What Matthew adds to Paul is essential.  The source of Paul's affirmation is not identified; as it stands in Romans, the claim of Paul about Jesus is rooted in faith, but it is not clear on what grounds that faith is based.  The source of affirmation in Matthew is very clear; it is divine revelation, assured by the greatest authority possible for human beings: God.  The  importance of this source of knowledge becomes clearer as Matthew's reader follows the attempts by Jesus' asssociates to identify him.  On the basis on what they experienced, most could call Jesus 'prophet', and a few could call him 'Messiah', though they did not know the real meaning of Jesus when they called him 'Messiah'.  Matthew will let that guessing go on throughout Jesus' public life, and it will be guessing because the crowds did not have a certitude about Jesus other than what has been mentioned.  The reader, however, is offered what Jesus' contemporaries did not have: an identification of Jesus which only divine revelation could give.  It is not human thinking, in Matthew's view, that is to accompany the adult Jesus through his famous public life; it is divine assurance about him that helps the reader to appreciate fully what others might only fleetingly suspect about Jesus from their experiences with him.  Paul's affirmation of Jesus, a valuable affirmation for its apostolic and Pauline truthfulness and conviction, yields to divine revelation in value, because the foundation of faith is most sure when God takes it upon Himself to give witness to what is true.