The Mystery of the Incarnation is defined as the Second Person of the Blessed Trinity (God the Son) becoming man as Jesus Christ—a zygote in the womb of the Blessed Virgin Mary. Mary is also known as the Mother of God and the Holy Theotokos.
Obviously, Mary was not the formal cause of the Incarnation, but she did more than just say “Yes” at the Annunciation. Consider that St. Augustine holds that Mary “conceived Christ in her mind before she did in her womb” (Christum prius mente quam ventre concipiens Sermo 215.4; PL 38:1074) and that first came “the Truth of Christ in Mary’s mind [and then] the flesh of Christ in Mary’s womb” (Veritas Christus in mente Mariae, caro Christus in ventre Mariae Sermo 25.7; PL 46: 937-938)
Now consider the fact that for the ancients, the heart was the seat of the intellect. There was no contradiction between heart and mind for a first century Jew, as there is for post-modern man. This means that Mary conceived Christ in her heart long before she conceived Him in her body. What could this possibly mean?
I don’t mean to fully imply the Incarnation was some type of spontaneous-combustion of charity in Mary’s soul (for we never want God to take a passive role in any Divine Mystery) but it’s not like Mary was randomly chosen outside of her already-supersonic levels of faith and hope and charity existing in her soul long, long before the Annunciation. Of course, all of this existed in her soul by God’s grace and her cooperation. At her Immaculate Conception (her conception in the womb of her mother, St. Anne) Mary already was at a level of charity far beyond what we can even hope to study (much less reach) in our ascetical theology ventures on earth.
If Christ was conceived in Mary’s heart long before He was in her womb (Christum prius mente quam ventre concipiens) then it is almost as if the Incarnation setting fire to the world was the only possible outcome of the incindiary union between the Holy Spirit and Mary her entire life from Immaculate Conception to the Annunciation in her youth, and beyond.
In fact, long before Mary was born, the angels were given a preview of it all. Bl. Mary of Agreda in her Mystical City of God writes that they were given a vision of Mary. Satan saw even before Mary was born that it would be within her that God would begin His hypostatic union (the union of the Divine Word’s Person with the humanity of Christ.) At this vision, Lucifer rebelled and became Satan, so offended was he at the fact that another creature (not Lucifer) would contain the hypostatic union of the Divine Person with His own sacred humanity. Ven. Mary of Agreda writes of the fall of angels: “Lucifer, however, and his confederates, rose to a higher pitch of pride and boastful insolence. In disorderly fury he aspired to be himself the head of all the human race and of the angelic orders, and if there was to be a hypostatic union, he demanded that it be consummated in him.”—Mystical City of God, p. 139.
Hugh of St. Victor writes: “Because the love of the Holy Spirit burned in her heart, that is why He worked wonderful things in her flesh, to the point that God and man would be born from her.” Do you see which came first between the interior and the exterior on the Incarnation? Mary’s “Yes” at the Annunciation had already been a perpetual “Yes” through her childhood. It was a “Yes” which was galaxies beyond what you or I have ever said to God on earth at even our best moment. This union existed from her infancy through her youth, teen years at the Incarnation, and beyond through the Assumption to today. God did not choose a random holy girl who just might say “Yes” at the Incarnation. The Incarnation almost had to happen in her, if, in fact, it were to happen at all. And by God’s love and mercy, the Incarnation did happen in real human history.