10 The Letter to the Ephesians, speaking of the "glory of grace" that "God, the Father...has bestowed on us in his beloved Son," adds: "In him we have redemption through his blood" (Eph. 1:7). According to the belief formulated in solemn documents of the Church, this "glory of grace" is manifested in the Mother of God through the fact that she has been "redeemed in a more sublime manner." By virtue of the richness of the grace of the beloved Son, by reason of the redemptive merits of him who willed to become her Son, Mary was preserved from the inheritance of original sin. In this way, from the first moment of her conception- which is to say of her existence-she belonged to Christ, sharing in the salvific and sanctifying grace and in that love which has its beginning in the "Beloved," the Son of the Eternal Father, who through the Incarnation became her own Son. Consequently, through the power of the Holy Spirit, in the order of grace, which is a participation in the divine nature, Mary receives life from him to whom she herself, in the order of earthly generation, gave life as a mother. The liturgy does not hesitate to call her "mother of her Creator" and to hail her with the words which Dante Alighieri places on the lips of St. Bernard: "daughter of your Son." And since Mary receives this "new life" with a fullness corresponding to the Son's love for the Mother, and thus corresponding to the dignity of the divine motherhood, the angel at the Annunciation calls her "full of grace."
Source: Redemptoris Mater (Vatican.va)