Catechism of the Catholic Church
502 The eyes of faith can discover in the context of the whole of Revelation the mysterious reasons why God in his saving plan wanted his Son to be born of a virgin. These reasons touch both on the person of Christ and his redemptive mission, and on the welcome Mary gave that mission on behalf of all men.
503 Mary's virginity manifests God's absolute initiative in the Incarnation. Jesus has only God as Father. "He was never estranged from the Father because of the human nature which he assumed. . . He is naturally Son of the Father as to his divinity and naturally son of his mother as to his humanity, but properly Son of the Father in both natures." 161
504 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." 162 From his conception, Christ's humanity is filled with the Holy Spirit, for God "gives him the Spirit without measure." 163 From "his fullness" as the head of redeemed humanity "we have all received, grace upon grace." 164
505 By his virginal conception, Jesus, the New Adam, ushers in the new birth of children adopted in the Holy Spirit through faith. "How can this be?" 165 Participation in the divine life arises "not of blood nor of the will of the flesh nor of the will of man, but of God". 166 The acceptance of this life is virginal because it is entirely the Spirit's gift to man. The spousal character of the human vocation in relation to God 167 is fulfilled perfectly in Mary's virginal motherhood.
506 Mary is a virgin because her virginity is the sign of her faith "unadulterated by any doubt", and of her undivided gift of herself to God's will. 168 It is her faith that enables her to become the mother of the Saviour: "Mary is more blessed because she embraces faith in Christ than because she conceives the flesh of Christ." 169
507 At once virgin and mother, Mary is the symbol and the most perfect realization of the Church: "the Church indeed. . . by receiving the word of God in faith becomes herself a mother. By preaching and Baptism she brings forth sons, who are conceived by the Holy Spirit and born of God, to a new and immortal life. She herself is a virgin, who keeps in its entirety and purity the faith she pledged to her spouse." 170
508 From among the descendants of Eve, God chose the Virgin Mary to be the mother of his Son. "Full of grace", Mary is "the most excellent fruit of redemption" (SC 103): from the first instant of her conception, she was totally preserved from the stain of original sin and she remained pure from all personal sin throughout her life.
509 Mary is truly "Mother of God" since she is the mother of the eternal Son of God made man, who is God himself.
510 Mary "remained a virgin in conceiving her Son, a virgin in giving birth to him, a virgin in carrying him, a virgin in nursing him at her breast, always a virgin" (St. Augustine, Serm. 186, 1: PL 38, 999): with her whole being she is "the handmaid of the Lord" (Lk 1:38).
511 The Virgin Mary "co-operated through free faith and obedience in human salvation" (LG 56). She uttered her yes "in the name of all human nature" (St. Thomas Aquinas, STh III, 30, 1). By her obedience she became the new Eve, mother of the living.
English Translation of the Cathechism of the Catholic Church for the United States of America © 1997, United States Catholic Conference, Inc.