56 The Father of mercies willed that the incarnation should be preceded by the acceptance of her who was predestined to be the mother of His Son, so that just as a woman contributed to death, so also a woman should contribute to life. That is true in outstanding fashion of the mother of Jesus, who gave to the world Him who is Life itself and who renews all things, and who was enriched by God with the gifts which befit such a role. It is no wonder therefore that the usage prevailed among the Fathers whereby they called the mother of God entirely holy and free from all stain of sin, as though fashioned by the Holy Spirit and formed as a new creature. Adorned from the first instant of her conception with the radiance of an entirely unique holiness, the Virgin of Nazareth is greeted, on God's command, by an angel messenger as "full of grace", and to the heavenly messenger she replies: "Behold the handmaid of the Lord, be it done unto me according to thy word". Thus Mary, a daughter of Adam, consenting to the divine Word, became the mother of Jesus, the one and only Mediator. Embracing God's salvific will with a full heart and impeded by no sin, she devoted herself totally as a handmaid of the Lord to the person and work of her Son, under Him and with Him, by the grace of almighty God, serving the mystery of redemption. Rightly therefore the holy Fathers see her as used by God not merely in a passive way, but as freely cooperating in the work of human salvation through faith and obedience. For, as St. Irenaeus says, she "being obedient, became the cause of salvation for herself and for the whole human race." Hence not a few of the early Fathers gladly assert in their preaching, "The knot of Eve's disobedience was untied by Mary's obedience; what the virgin Eve bound through her unbelief, the Virgin Mary loosened by her faith." Comparing Mary with Eve, they call her "the Mother of the living," and still more often they say: "death through Eve, life through Mary."
Source: Lumen Gentium (Vatican.va)