A feast called the Conception of Mary arose in the Eastern Church in the seventh century. It came to the West in the eighth century. In the 11th century it received its present name, the Immaculate Conception. In the 18th century it became a feast of the universal Church. It is now recognized as a solemnity.
In 1854, Pius IX solemnly proclaimed: “The most Blessed Virgin Mary, in the first instant of her conception, by a singular grace and privilege granted by almighty God, in view of the merits of Jesus Christ, the savior of the human race, was preserved free from all stain of original sin.”
It took a long time for this doctrine to develop. While many Fathers and Doctors of the Church considered Mary the greatest and holiest of the saints, they often had difficulty in seeing Mary as sinless—either at her conception or throughout her life. This is one of the Church teachings that arose more from the piety of the faithful than from the insights of brilliant theologians. Even such champions of Mary as Bernard of Clairvaux and Thomas Aquinas could not see theological justification for this teaching.
Two Franciscans, William of Ware and Blessed John Duns Scotus, helped develop the theology. They pointed out that Mary’s Immaculate Conception enhances Jesus’ redemptive work. Other members of the human race are cleansed from original sin after birth. In Mary, Jesus’ work was so powerful as to prevent original sin at the outset.
“[Mary] gave to the world the Life that renews all things, and she was enriched by God with gifts appropriate to such a role.
“It is no wonder, then, that the usage prevailed among the holy Fathers whereby they called the mother of God entirely holy and free from all stain of sin, fashioned by the Holy Spirit into a kind of new substance and new creature. Adorned from the first instant of her conception with the splendors of an entirely unique holiness, the Virgin of Nazareth is, on God’s command, greeted by an angel messenger as ‘full of grace’ (cf. Luke 1:28). To the heavenly messenger she replies: ‘Behold the handmaid of the Lord, be it done to me according to thy word’ (Luke 1:38)” (Vatican II, Dogmatic Constitution on the Church, 56).