December 8th 1854 marked Pope Pius IX’s official dogmatisation of the doctrine of Immaculate Conception; an important act, as it is often the key belief that separates Catholicism and Protestantism, among other doctrines. Such an act had been hinted at previously, by such popes as Pope Sixtus IV, who declared the feat of the Immaculate Conception in 1476, and the Council of Trent allowed the existence of the doctrine, but prior to Pius IX’s announcement, the doctrine had never existed as dogma.
In essence, the belief holds that God allowed the Virgin Mary to be born without Original Sin – the sin that each human after Adam and Eve is born into. Original Sin itself is a contentious doctrine, one that is characteristically Catholic, hence the belief that unbaptised babies will go to Limbo. Nevertheless the Catholic doctrine of Immaculate Conception dictates that Mary was born without such sin, allowing her to be the mother of Jesus, and subsequently to lead a sinless life.
Outside Catholicism, the belief is generally not a common Christian belief. Orthodox sects do not believe in original sin, however they do believe that Mary led a sinless life. On the other hand, most protestant and affiliated sects commonly reject both the doctrine of Immaculate Conception and Original Sin. This is based on Paul’s teachings in the New Testament that the law exists to outline sin, and thus sin does not exist outside of law. Since humans are not born into law itself, they are not born into sin. Protestants also commonly believe that Mary was and is not such a key figure in Christianity on the same level that Catholics hold to.
December 8 is celebrated as the Feast of the Immaculate Conception in Catholicism.