In Mary, maternity, because it is divine, goes beyond mere human maternity. The Son of Mary only began His humanity in her. It was not she who “made” her Son, it was He who made His mother His own. Mary gave a human nature to God, but it is God who received it, who comes to take it. The result is a new relationship, more than maternal.
The fiat is a response to the gift that the divine Person makes of Himself. If Mary must consent spiritually to what happens in her flesh, it is not only to the birth of this new being to which she must consent, it is to the Incarnation of God.
And due to being His mother, it is to make Him a man, to be His mother is to be united with the intention for which the Word was made flesh, to become man. The fundamental union between Mary and her Son is this total unity of will in the very act of the Incarnation. A bond is therefore formed between the two of them which has no equivalent in purely human maternity.
Mother and spouse
It is traditional to use the image of the spouse to explain the relationship of Yahweh with His people, of the Incarnate Word with the Church and even with the soul of each of the faithful. It must be understood on a spiritual and mystical level.
Such a union was eminently realized between Mary and the Incarnate Word. St. Thomas speaks of the fiat as a “nuptial consent,” of the mutual gift that constitutes for him the essence of marriage.
In a human marriage, the consent relates to the union of bodies and lives. What corresponds to it in the Incarnation, is the union of the two natures in Christ, but also of the entire human race included in the human nature of Christ, with the divinity. If Mary’s consent is nuptial, it is given in the name of the human race.
It was highly appropriate that the Virgin consent to the Incarnation through the fiat
To show the extent of Mary’s union with God. If the divine Maternity achieves the closest union with God, it must involve this teaching that will allow the Virgin to conceive the divine Word in her heart before conceiving Him in her flesh, as St. Leo the Great says.
For the benefit of her faith. Thanks to the Annunciation, the Virgin was more certain of the mystery of which she was to bear witness. She was too humble to believe and say these grandiose things about herself. This appearance gave her a more sure faith in the reality of this mystery.
On the side of human nature. The Incarnation is realized as a spiritual marriage between the Word and human nature. It is thus necessary for each of the parties to give his consent to this marriage. Mary represents human nature in this exchange.
The fiat is given especially in the name of Mary. The Virgin adheres to the Incarnation in all its unfolding, she is united body and soul to all the sentiments living in the heart of her Son since His conception. St. Bernard has magnificently described this scene:
“We too, are waiting, O Lady, for a word of compassion; the sentence of condemnation weighs heavily upon us. The price of our salvation is offered to you; if you consent, we shall at once be delivered.”
“The whole earth waits for this response, prostrate at your feet. For on your word depend the consolation of the wretched, the salvation of your entire race, of all the sons of Adam.”
“Open, O Blessed Virgin, your heart to faith, your lips to compliance, your womb to the Creator. Arise in faith, run in haste to His will, open it with your consent.”
Sermon on the Praises of the Virgin Mary, St. Bernard of Clairvaux