From all eternity Father, Son, and Holy Spirit chose Mary to be the Mother of the Word Incarnate. It was unthinkable that she be defiled by sin, in any way or for the least second of time. Therefore, in the instant of creating Mary's person, the Trinity made her immaculate. This grace, like every other grace granted since the fall of Adam, was given her through the merits of Christ. She was truly saved, redeemed, not from an evil already present, but from an evil that threatened.
~ Introduction to the Mass text
O God, Who by the foreseen merits of the death of Christ, You shielded Mary from all stain of sin and preserved the Virgin Mother immaculate at her conception so that she might be a fitting dwelling place for Your Son. Cleanse us from sin through her intercession so that we also may come to You untainted by sin.
~ Collect from the Mass for the Immaculate Conception
O Mary, conceived without sin, Patroness of the United States of America, Pray for us, intercede for our country!