How can Catholics believe Mary was conceived without original sin when Romans 3:23 admonishes us that “all have sinned and are deprived of the glory of God”?
St. Paul is making a general statement about the state of humanity after the fall. He does not intend to exclude exceptions to the “all” who have sinned, because Jesus never sinned nor did the good angels. In 1 Corinthians 4:5, St. Paul expresses that “everyone will receive praise from God.” Does this “everyone” include Satan, the demons and the damned? In fact, the Catholic dogma of Mary’s Immaculate Conception developed precisely because Mary is the mother of Jesus, who “has similarly been tested in every way, yet without sin” (Heb 4:15).
The dogma of Mary’s plenitude of grace from the time of her conception finds support in the greeting of the angel in Luke 1:28. Mary is spoken of as “full of grace” (kekaritomene) which in Greek means one who has been and continues to be “favoured” or “graced” by God (alternate translations could be “totally graced” or “completely graced”). Since Jesus is completely human but without sin, it was essential, then, that he takes his human nature from one who is “full of grace” and thus free from all sin (original as well as personal sin).
In the 13th century, the Franciscan theologians William of Ware and Blessed John Duns Scotus developed the idea of “anticipatory redemption” or “preredemption.” The merits of Christ were applied to Mary in waiting for her future role as the mother of the Word Incarnate. Thus, Mary was redeemed by Christ by being preserved from all stain of original sin. In 1854, Blessed Pius IX defined the doctrine of the Immaculate Conception as a truth made known by God to be believed by all the faithful.