Mary became the Mother of God at the moment of the Incarnation. It was a moment for which God had richly prepared her. Through the fullness of grace which He gave her, Mary had lived a life of obedience to the will of God. Her every thought and action had been formed in the burning crucible of charity or love of God. She had even conceived the resolution to devote herself to the service of God by a vow of virginity. At the moment chosen by God, the Archangel Gabriel appeared to her in bodily form and announced to her the great purpose for which God had chosen her. "Hail, full of grace", he said, and his words signified her worthiness to fulfill the role for which she was destined. "The Lord is with thee," he continues, and in these words he announced the conception that was to take place. When Mary gave the full consent of her loving, obedient heart in the words, "Be it done unto me according to thy word", the marriage between God and human nature was complete. God had become man, and in the consent of Mary all mankind consented to its own ennoblement in the God-man, Christ. Eve seduced Adam to the destruction of the human race. Mary conceived Christ for the salvation of the human race.
Taken from "The Summa simplified" chapter 'Mary: the Virgin Mother of God'.