Development of the Doctrine:
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.
Spread of the Feast in the West:
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, however, 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.
Promulgation of the Dogma of the Immaculate Conception:
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."