Immaculate Conception. In Christian theology, the Immaculate Conception is the conception of the Virgin Mary free from original sin by virtue of the merits of her son Jesus. The Catholic Church teaches that God acted upon Mary in the first moment of her conception, keeping her immaculate. The Immaculate Conception is commonly confused with the virgin birth of Jesus, the latter being, rather, the doctrine of the Incarnation. While many Christians believe in the virgin birth of Jesus, it is principally Roman Catholics, along with various other Christian denominations, who believe in the doctrine of the Immaculate Conception. Although the belief that Mary was sinless, or conceived without original sin, has been widely held since Late Antiquity, the doctrine was not dogmatically defined in the Catholic Church until 1854 when Pope Pius IX, declared ex cathedra, i.e., using papal infallibility, in his papal bull Ineffabilis Deus, the Immaculate Conception to be doctrine. The Catholic Church celebrates the Solemnity of the Immaculate Conception on December 8; in many Catholic countries, it is a holy day of obligation or patronal feast, and in some a national public holiday. 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. The Encyclical Mystici Corporis from Pope Pius XII 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.