The Feast of the Immaculate Conception, in its oldest form, goes back to the seventh century, when churches in the East began celebrating the Feast of the Conception of Saint Anne, the mother of Mary. In other words, this feast celebrates the conception of the Blessed Virgin Mary in the womb of Saint Anne; and nine months later, on September 8, we celebrate the Nativity of the Blessed Virgin Mary.
As originally celebrated (and as still celebrated in the Eastern Orthodox Churches), however, the Feast of the Conception of Saint Anne does not have the same understanding as the Feast of the Immaculate Conception has in the Catholic Church today. The feast arrived in the West probably no earlier than the 11th century, and at that time, it began to be tied up with a developing theological controversy. Both the Eastern and the Western Church had maintained that Mary was free from sin throughout her life, but there were different understandings of what this meant.
Because of the doctrine of Original Sin, some in the West began to believe that Mary could not have been sinless unless she had been saved from Original Sin at the moment of her conception (thus making the conception "immaculate"). Others, however, including St. Thomas Aquinas, argued that Mary could not have been redeemed if she had not been subject to sin—at least, to Original Sin.
The answer to St. Thomas Aquinas's objection, as Blessed John Duns Scotus (d. 1308) showed, was that God had sanctified Mary at the moment of her conception in His foreknowledge that the Blessed Virgin would consent to bear Christ. In other words, she too had been redeemed—her redemption had simply been accomplished at the moment of her conception, rather than (as with all other Christians) in Baptism.
After Duns Scotus's defense of the Immaculate Conception, the feast spread throughout the West, though it was still often celebrated at the Feast of the Conception of Saint Anne. On February 28, 1476, Pope Sixtus IV extended the feast to the entire Western Church, and in 1483 threatened with excommunication those who opposed the doctrine of the Immaculate Conception. By the middle of the 17th century, all opposition to the doctrine had died out in the Catholic Church.
On December 8, 1854, Pope Pius IX officially declared the Immaculate Conception a dogma of the Church, which means that all Christians are bound to accept it as true. As the Holy Father wrote in the Apostolic Constitution Ineffabilis Deus, "We declare, pronounce, and define that the doctrine which holds that the most Blessed Virgin Mary, in the first instance 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, is a doctrine revealed by God and therefore to be believed firmly and constantly by all the faithful."
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