Although evil had dominion over the entire inhabited earth and had everywhere wrought confusion, commotion, and havoc, it was defeated by a single thought and a single soul, and it yielded not only to her, but also, on account of her, to the entire human race.
[…] Since that time had now come and the Angelic messenger was at hand, she believed, gave her consent, and undertook her ministry. These things were indispensable and in every way necessary for our salvation; without them, there would have been no hope for humanity.
For, neither would it have been possible, had the Blessed Virgin not prepared herself, as I said, for God to look kindly on mankind and to desire to descend to earth, that is, had there not been someone to receive Him, someone capable of serving Him in the œconomy of salvation; nor would it have been possible, had she not believed and given her consent, for God’s will for us to have been realized.
This is evident from the fact that Gabriel, in addressing the Virgin and calling her “Full of Grace,” expressed everything pertaining to the mystery. God did not descend until the Virgin sought to learn the manner of her conceiving. But when He saw that she was persuaded and that she accepted the invitation, the deed was accomplished straightway; and God clothed Himself in humanity and the Virgin became the Mother of her Creator.
In the case of Adam, God neither foretold nor persuaded him concerning the rib from which Eve was to be fashioned, but put him to sleep, and in this way deprived him of the member in question; in the case of the Virgin, however, He first instructed her and awaited her assurance before proceeding to the deed.
Regarding the creation of Adam, He conversed with His Only-Begotten Son, saying: “Let Us make man” (Gen. 1:26). But when, as Paul says, He was going to bring this wonderful Counselor (Isa. 9:6), the First-Begotten, into the world (Heb. 1:6), and to form the second Adam, He made the Virgin a participant in his decision. And this great counsel, about which Isaiah speaks, God proclaimed and the Virgin ratified.
The Incarnation of the Word was the work not only of the Father, Whose good pleasure it was, and of His Power (1 Cor. 1:24), Who overshadowed, and of His Spirit, Who descended, but also of the will and faith of the Virgin. For, just as, without those Three, it would have been impossible for this decision to be implemented, so also, if the All-Pure One had not offered her will and faith, this design could not possibly have been brought to fruition.
Nicholas Cabasilas (1319/1323–after 1391): On the Occasion of the Feast of the Annunciation, 3-4, Translated from the Greek text in “Homélies Mariales Byzantines (II),” ed. M. Jugie, in Patrologia Orientalis, Vol. XIX, pp. 484-495@ Old Calendar Orthodox Church of Greece.