9 If the greeting and the name "full of grace" say all this, in the context of the angel's announcement they refer first of all to the election of Mary as Mother of the Son of God. But at the same time the "fullness of grace" indicates all the supernatural munificence from which Mary benefits by being chosen and destined to be the Mother of Christ. If this election is fundamental for the accomplishment of God's salvific designs for humanity, and if the eternal choice in Christ and the vocation to the dignity of adopted children is the destiny of everyone, then the election of Mary is wholly exceptional and unique. Hence also the singularity and uniqueness of her place in the mystery of Christ. The divine messenger says to her: "Do not be afraid, Mary, for you have found favor with God. And behold, you will conceive in your womb and bear a son, and you shall call his name Jesus. He will be great, and will be called the Son of the Most High" (Lk. 1:30-32). And when the Virgin, disturbed by that extraordinary greeting, asks: "How shall this be, since I have no husband?" she receives from the angel the confirmation and explanation of the preceding words. Gabriel says to her: "The Holy Spirit will come upon you, and the power of the Most High will overshadow you; therefore the child to be born will be called holy, the Son of God" (Lk. 1:35). The Annunciation, therefore, is the revelation of the mystery of the Incarnation at the very beginning of its fulfillment on earth. God's salvific giving of himself and his life, in some way to all creation but directly to man, reaches one of its high points in the mystery of the Incarnation. This is indeed a high point among all the gifts of grace conferred in the history of man and of the universe: Mary is "full of grace," because it is precisely in her that the Incarnation of the Word, the hypostatic union of the Son of God with human nature, is accomplished and fulfilled. As the Council says, Mary is "the Mother of the Son of God. As a result she is also the favorite daughter of the Father and the temple of the Holy Spirit. Because of this gift of sublime grace, she far surpasses all other creatures, both in heaven and on earth."
Source: Redemptoris Mater (Vatican.va)