In her Immaculate Conception, Mary was preserved from the stain of original sin. When he at long last defined the Church’s ancient – yet undeclared – understanding of Mary’s Immaculate Conception, Pope Pius IX said:
From the very beginning, and before time began, the eternal Father chose and prepared for his only-begotten Son a Mother in whom the Son of God would become incarnate and from whom, in the blessed fullness of time, he would be born into this world. Above all creatures did God so love her that truly in her was the Father well pleased with singular delight. Therefore, far above all the angels and all the saints so wondrously did God endow her with the abundance of all heavenly gifts poured from the treasury of his divinity that this mother, ever absolutely free of all stain of sin, all fair and perfect, would possess that fullness of holy innocence and sanctity than which, under God, one cannot even imagine anything greater, and which, outside of God, no mind can succeed in comprehending fully (Ineffabilis Deus).Mary, as the perfect disciple of Christ and exemplar of the Church, was conceived and born in the state of grace, as God intended for his creation from the very beginning. “It was Mary’s privilege to receive this gift, not returned because never lost, but given in virtue of the redeeming blood of her Son, in order to prepare a place for him.”
Only by being preserved free from the stain of sin could Mary truly be a fitting and worthy temple and vessel of the Lord. Precisely because of her Immaculate Conception Mary was the true Tabernacle, the Dwelling of the Lord Most High. This is Mary’s singular honor.
We know that the stain of original sin, which we inherited from the sin of Adam and Eve, excluded us from Paradise, from eternal life with the Father. “Each of us must once more find entry to the paradise lost from which Mary was never excluded, entry to the eternal company of God, presented, as St. Jude says, ‘spotless before the presence of His glory with exceeding joy.’” Mary, then, shows us the way by showing us her Son.
Because of her sin Eve was banished from Paradise, and we with her. Because of her absolute preservation from sin Mary was never excluded from Paradise. Death entered the world through the sin of Eve but through Mary “the way, the truth and the life” entered the world. Eve is the mother of all the living while Mary is the Mother of all of the redeemed. Just as Jesus Christ is the new Adam, so is Mary the new Eve.
Indeed, Mary is the woman of whom the LORD God said to the serpent: “I will put enmity between you and the woman, and between your offspring and hers; he will strike at your head, while you strike at his heel” (Genesis 3:20). While the devil strikes at Mary’s heel, Mary – and the Church with her – stomps at his head.
Through her acceptance of the will of the Lord Mary – and the Church – has triumphed over the serpent. Victory is ours through the precious blood of Christ, and yet our battle with the serpent in this life must still be fought. Together with Saint Bernard of Clairvoux and with Adam, we ought to cry out:
The woman, whom you gave me to be my companion, gave me of the tree, and I did eat. Make haste now, O Eve, and come to Mary; come O Mother, to your Daughter! Change those words from a miserable excuse to a song of thanksgiving: Lord, the woman, whom you gave me to be my companion, gave me to eat from the tree of life!Mary takes us to her Son and says to us: “Eat of his Body, drink of his Blood. You will find life and he will make you spotless and immaculate.” “We are sure that, from on high, Mary follows our steps with gentle trepidation, gives us serenity in the hour or darkness and storm, and gives us security with her maternal hand” (Pope Benedict XVI, General Audience, 16 August 2006). Amen.
 Hugo Rahner, S.J. Our Lady and the Church. Sebastian Bullough, O.P., trans. (Bethesda, MD: Zaccheus Press, 2004), 20.
 Homily 2 on “Missus est” (PL 183, 62).