Catechism of the Catholic Church

Jesus is conceived by the Holy Spirit in the Virgin Mary's womb because he is the New Adam, who inaugurates the new creation: "The first man was from the earth, a man of dust; the second man is from heaven." From his conception, Christ's humanity is filled with the Holy Spirit, for God "gives him the Spirit without measure." From "his fullness" as the head of redeemed humanity "we have all received, grace upon grace." (359)