From "Our Lady in the Gospel" by Venerable John Henry Newman, C.O.
" The Holy Fathers have ever gathered the exact obedience and the sinlessness of the Blessed Virgin from the very narrative of the Annunciation, when she became the Mother of God. For when the Angel appeared to her and declared to her the will of God, they say that she displayed especially four graces, humility, faith, obedience and purity. Nay, these graces were as it were, preparatory conditions to her being made the minister of so high a dispensation. So that if she had not had faith, and humility, and purity, and obedience, she would not have merited to be God's Mother. Thus it is common to say that she conceived Christ in mind before she conceived Him in body, meaning that the blessedness of faith and obedience preceded the blessedness of being a Virgin Mother. Nay, they even say that God waited for her consent before He came into her and took flesh of her. Just as He did no mighty works in one place because they had not faith, so this great miracle, by which He became the Son of a creature, was suspended till she was tried and found meet for it—till she obeyed. "
Faith and Prejudice and Other Unpublished Sermons