Man refused to love God when he was tempted by the devil and allowed himself to be deceived by him. Since man fell into sin out of weakness, God decreed in His eternal Providence not to allow the work of His hands to suffer eternal ruin.
He who had “established” the universe “in wondrous dignity”, resolved to restore it “even more admirably” (offertory of the Holy Mass).
The sin of our first parents had punishments as its necessary consequence, but at the moment when they were driven out of paradise and lost immortality and their friendship with God, God promised mankind a future redemption, specifically, that the day would come when Satan’s bonds would be loosed.
“I will put enmities between thee and the Woman, and thy seed and her seed: she shall crush thy head and thou shalt lie in wait for her heel” (Genesis 3:15).
If mankind had not obtained this light, they would have despaired. So instead this first glad tiding was given to them at the very beginning (and that is why this promise is called the Protoevangelium), and this first hope is the Woman and her Offspring, Mary and her Divine Son.
Pope Pius IX confirms this in the encyclical Ineffabilis Deus: “These ecclesiastical writers [the Fathers and Doctors of the Church] in quoting the words by which at the beginning of the world God announced His merciful remedies prepared for the regeneration of mankind—words by which He crushed the audacity of the deceitful serpent and wondrously raised up the hope of our race, saying:
‘I will put enmities between you and the woman, between your seed and her seed’—taught that by this divine prophecy the merciful Redeemer of mankind, Jesus Christ, the only-begotten Son of God, was clearly foretold: That His most Blessed Mother, the Virgin Mary, was prophetically indicated; and, at the same time, the very enmity of both against the evil one was significantly expressed.
“Hence, just as Christ, the Mediator between God and man, assumed human nature, blotted the handwriting of the decree that stood against us, and fastened it triumphantly to the cross, so too the most holy Virgin, united with Him by a most intimate and indissoluble bond, was, with Him and through Him, eternally at enmity with the evil serpent, and most completely triumphed over him, and thus crushed his head with her immaculate foot.”
Of all the prophets, Isaiah describes most clearly the Savior who is to come, and he promises that he will be the son born of a virgin (Isaiah 7:14). The wisdom literature of the Old Testament praises the woman who is to come as God’s “beloved” (Canticle of Canticles 2:16) and as “the king’s daughter” (Psalm 44:14).
The Woman who will crush the serpent “cometh forth as the morning rising, fair as the moon, bright as the sun, terrible as an army set in array” (Canticle of Canticles 6:9). “Thou art all fair, O my love, and there is not a spot in thee” (Canticle of Canticles 4:7). Thus, from the very beginning, Our Lady is at the focal point of the promises.
Actually God could have redeemed the world in any one of many possible ways. What matters for us is the reality, namely the way that He in fact chose.
“God has decided to begin and accomplish His greatest works through the Blessed Virgin ever since He created her, [and] we can safely believe that He will not change His plan in the time to come, for He is God and does not change in His thoughts or His way of acting.” (Saint Louis Mary Grignion de Montfort)