The Immaculate Conception is a Roman Catholic doctrine that the Virgin Mary was preserved by God from the transmission of original sin at the time of her own conception. It is not, as is popularly believed, another name for the doctrine of the virgin birth. Immaculate Conception was defined by Pope Pius IX in his constitution Ineffabilis Deus, published December 8, 1854 (the Feast of the Immaculate Conception).
The doctrine is generally not shared by either Eastern Orthodoxy or by Protestantism, though for different reasons. Eastern Orthodoxy does not share this doctrine because it does not share Catholicism's Augustinian view of original sin and total depravity, and consequently considers the doctrine unnecessary. There are numerous references in the Greek and Syrian Fathers to Mary's purity and sinlessness, but it has not been claimed by Orthodox theologians that this refers to an a priori state and could correctly refer to her conduct after birth. In addition, there is a theological argument that, had the Theotokos been somehow rendered sinless before Jesus was Incarnated, then redemption of humanity in general would not have occurred, as the Virgin Mary was to have been the "first fruits of salvation", the first who would be cleansed of the effects of Original Sin by means of Christ's incarnation, death and resurrection. Were she already untainted, she could not truly fulfill this necessary function.
Protestantism rejects the doctrine because it is not explicitly spelled out in the Bible. They also argue that if Mary could be born without sin, there does not appear to be any need for Jesus to be incarnated and die to save humanity from sin. Some Orthodox theologians likewise raise the second objection. Catholics respond to this argument by pointing out that since Jesus became incarnate of the Virgin Mary, she needed to be completely free of sin to bear the Son of God. Mary's freedom from sin depended entirely on the grace of God, and she too was redeemed by the saving merits of Christ, yet, of necessity, these merits were applied to her "in advance".