The defined dogma of the Immaculate Conception states:
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 Saviour 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.
Declaramus, pronuntiamus et definimus doctrinam, quae tenet, beatissimam Virginem Mariam in primo instanti suae Conceptionis fuisse singulari omnipotentis Dei gratia et privilegio, intuitu meritorum Christi lesu Salvatoris humani generis, ab omni originalis culpae labe praeservatam immunem, esse a Deo revelatam, atque idcirco ab omnibus fidelibus firmiter constanterque credendam. Quapropter si qui secus ac a Nobis.
The definition concerns original sin only, and it makes no declaration about the Church's belief that the Blessed Virgin was sinless in the sense of freedom from actual or personal sin.
The doctrine teaches that from her conception Mary, being always free from original sin, received the sanctifying grace that would normally come with baptism after birth.
Possibility of personal sin
The Encyclical Mystici Corporis from Pope Pius XII (1943) in addition holds that Mary was also sinless personally, "free from all sin, original or personal". In this, Pius XII repeats a position already expressed by the Council of Trent, which decreed "If anyone shall say that a man once justified can sin no more, nor lose grace, and that therefore he who falls and sins was never truly justified; or, on the contrary, that throughout his whole life he can avoid all sins even venial sins, except by a special privilege of God, as the Church holds in regard to the Blessed Virgin: let him be anathema."
When defining the dogma in Ineffabilis Deus, Luke 1:47, Mary proclaims: "My spirit has rejoiced in God my Saviour." This is referred to as Mary's pre-redemption by Christ. Since the Second Council of Orange against semi-pelagianism, the Catholic Church has taught that even had man never sinned in the Garden of Eden and was sinless, he would still require God's grace to remain sinless.
The doctrine of the immaculate conception (Mary being conceived free from original sin) is not to be confused with the virginal conception of her son Jesus. Catholics believe that Mary was conceived of both parents, traditionally known by the names of Saint Joachim and Saint Anne. In 1677, the Holy See condemned the error of Imperiali who taught that St. Anne in the conception and birth of Mary remained virgin, which had been a belief surfacing occasionally since the 4th century. The Church celebrates the Feast of the Immaculate Conception (when Mary was conceived free from original sin) on 8 December, exactly nine months before celebrating the Nativity of Mary. The feast of the Annunciation (which commemorates the virginal conception and the Incarnation of Jesus) is celebrated on 25 March, nine months before Christmas Day.