Why is Mary's yes to God so important to us?

Through Mary’s fiat, her “yes” to God, she made the Divine Plan of salvation effective in history. As Eve disobeyed and set humanity on a path of sin, Mary was perfectly obedient and set us on the path of grace. As Eve was the “Mother of All the Living” (Gen. 3:20), Mary is the Mother of All those Living “in Christ.” We owe her a debt of justice, and a filial obligation of love.

How did Mary respond to God's request?

Mary responded with the most perfect faith and obedience.

How does Mary serve as the new Eve in our salvation story?

St. Irenaeus taught,

Just as the former—that is, Eve—was seduced by the words of an angel so that she turned away from God by disobeying his word, so the latter—Mary— received the good news from an angel's announcement in such a way as to give birth to God by obeying his word; and as the former was seduced so that she disobeyed God, the latter let herself be convinced to obey God, and so the Virgin Mary became the advocate of the virgin Eve. And as the human race was subjected to death by a virgin, it was liberated by a Virgin; a virgin's disobedience was thus counterbalanced by a Virgin's obedience….