The Roman Catholic doctrine of the Immaculate Conception of the Blessed Virgin Mary, as defined by Pope Pius IX in 1854, states that "in the first instance of her conception, by a singular privilege and grace granted by God, in view of the merits of Jesus Christ, the Saviour of the human race, was preserved exempt from all stain of original sin." Thus, God in eternity, outside of time, intervened at the moment of her conception by applying to her the grace of salvation in his Son Jesus Christ, "slain from the foundation of the world" (1 Peter 1:19-20; Revelation 13:8): "my spirit rejoices in God my Savior" (Luke 1:47). Had God not intervened with unmerited salvation, she would immediately have had the defect of original sin.
The Feast of the Immaculate Conception is celebrated on 8 December.
The Immaculate Conception is often erroneously linked to the Conception of Jesus rather than Mary. Jesus would have had no need for an Immaculate Conception, assuming his deity. According to this doctrine, Mary, being human, was given the grace of perfection before her birth so as to serve as the vessel of the Lord.
The medieval Catholic philosopher and theologian, Duns Scotus, famously argued for this doctrine as follows: Potuit, decuit, ergo fecit, which is Latin for God could do it, it was appropriate that God do it, therefore God did it. Scotus argued that since God could have caused Mary to be miraculously conceived free of original stain, and it was fitting for God to do so, therefore God must have so done.
Protestant positions on this topic differ. They believe Mary to be a normal human being, a sinner, who was used by God to bring Christ into the world. This is a reasonable point of view given the lack of Biblical support for Mary being "exempt from all stain of original sin." The primary Biblical evidence cited by Catholics is the passaage in which the angel announces to Mary that she would be the mother of the Savior and uses these words, "Hail, Mary, full of grace...."
Does "full" mean sinless, and if so, does it mean that she always was so? Unless the answer to the latter question is "yes," the doctrine of the Immaculate Conception is not affirmed. Hebrews 10:5 says, "a body thou hast prepared for me", which has been applied to the immaculate conception of the body of Mary in advance preparation for the incarnation of Jesus, so that at his incarnation he would assume his sinless flesh from her body. The same passage has also been limited as applying instead only to the moment of Jesus' incarnation in her womb, so that the flesh of his body alone was preserved free of sin.
Pertinent Scripture texts used by Protestant Christians in opposition to the Roman view include:
"For there is one God, and one mediator between God and men, the man Christ Jesus; Who gave himself a ransom for all..." I Tim. 2:5-6
"For all have sinned, and come short of the glory of God." Rom. 3:23 (emphasis added)
"And ye know that he was manifested to take away our sins; and in him is no sin." I John 3:5 (emphasis added)
"If we say that we have no sin, we deceive ourselves, and the truth is not in us." I John 1:8
"Neither is there salvation in any other: for there is none other name (Jesus Christ) under heaven given among men, whereby we must be saved." Acts 4:12
These passages allow the possibility that not even Mary could have been without blemish at some time in her life. However, "all have sinned" has also been read as applying instead to the collective whole of the human race, not absolutely to every single individual human being; otherwise not even Jesus as a member of the human race could be exempt and without blemish at some time in his life, for the same strictly literal reasoning from these passages as applied to Mary allow this possibility too, making Romans 3:23 and I John 1:8 contradict I John 3:5.