The Solemnity of the Immaculate Conception celebrates Mary's conception, without sin. The Immaculate Conception is a uniquely Catholic feast, although the Eastern Church celebrates Mary's conception. The feast falls on December 8th in the West and December 9 in the East. Prayers: Immaculate Conception Prayers
We declare, pronounce and define that the doctrine which asserts that the Blessed Virgin Mary, from the first moment of her conception, by a singular grace and privilege of almighty God, and in view of the merits of Jesus Christ, Saviour of the human race, was preserved free from every stain of original sin is a doctrine revealed by God and, for this reason, must be firmly and constantly believed by all the faithful.
With these words, spoken in 1854, Pope Pius IX, in the Papal Bull ineffabilis Dei, declared Mary's Immaculate Conception to be dogma. Pius did not invent the concept. Rather, he was affirming a belief held by many Christians that came before him, from East and West, that Mary was conceived free of the stain of original sin, on account of Christ's work. God caused this immaculate conception in order to render Mary a pure vessel to bear God-made-flesh.
Mary, the one who is "full of grace" and the one whom "all generations will called 'blessed'" has been viewed as unique since the earliest days of the Christian faith. Just as Christ has been called the "new Adam," the Church Fathers, especially Saints Justin (AD 150) and Irenaeus (AD 180), saw Mary as the "new Eve," who humbly obeyed God, even though Eve disobeyed. The Church Fathers also called Mary the "new ark of the covenant" and theotokos, God-bearer. It is from these titles that the doctrine of Mary's Immaculate Conception and sinlessness unfolded. Thus St. Ephrem the Syrian (d. AD 373) spoke of Mary as without stain or blemish, calling her "all-pure, all-immaculate, all-stainless, all-undefiled, all-incorrupt, all-inviolate" (see Nisibine Hymns, and "Precationes ad Deiparam"). St. Ambrose (d. AD 397) wrote "lift me up not from Sarah, but from Mary, a Virgin not only undefiled, but a Virgin whom grace has made inviolate, free of every stain of sin" (Commentary on Psalm 118). Augustine left open the possibility of Mary's sinlessness, even using language similar to the Catholic dogma of the Immaculate Conception:
We must except the holy Virgin Mary, concerning whom I wish to raise no question when it touches the subject of sins, out of honour to the Lord; for from Him we know what abundance of grace for overcoming sin in every particular was conferred upon her who had the merit to conceive and bear Him who undoubtedly had no sin (On Nature and Grace, 42).
Later Fathers, such as St. John of Damascus (d. AD 755) and St. Andrew of Crete (d. AD 740) continued this emphasis on Mary's sinlessness as bearer of God. John of Damascus wrote:
The Father's...sanctifying power overshadowed her, cleansed and made her holy, and, as it were, predestined her. Then Thou, Word of the Father...didst take flesh of the Blessed Virgin, vivified by a reasoning soul, having first abided in her undefiled and immaculate womb...(Sermon I: On the Assumption)
John also spoke of Mary's "holy, undefiled, and stainless soul" (Sermon II: On the Assumption). However, there was no official dogma of the Immaculate Conception as of this period. Most Church Fathers agreed that Mary was sinless at the time she gave birth to Christ. They disagreed as to whether Mary was made sinless at conception, birth, or when she said "yes" to God's call. Even some prominent medieval Western theologians (notably St. Bonaventure and St. Thomas Aquinas*) denied Mary's Immaculate Conception, although not her sinlessness. Even today, Catholic and Orthodox theologians agree that Mary is the all-holy, blameless, "new ark"; the debate is not about Mary being sinless, but about when Mary was made sinless. Part of this disagreement is because the East does not believe in original sin as the Western Church defines it. Orthodox theologian John Myendorff, in Byzantine Theology, has suggested the East would likely accept the Catholic dogma of the Immaculate Conception if they had a Western understanding of Original Sin. The East and West nonetheless seem to be getting at the same "mystery": Mary's sinlessness and holiness in her role as theotokos.