To the shepherds, the angel announced the birth of Jesus as the Messiah promised to Israel: "To you is born this day in the city of David a Saviour, who is Christ the Lord." From the beginning he was "the one whom the Father consecrated and sent into the world", conceived as "holy" in Mary's virginal womb. God called Joseph to "take Mary as your wife, for that which is conceived in her is of the Holy Spirit", so that Jesus, "who is called Christ", should be born of Joseph's spouse into the messianic lineage of David.
Source: Catechism of the Catholic Church (Vatican.va)