Definition of the Immaculate Conception of the Blessed Virgin Mary
The following is the definition of the Blessed Virgin Mary, given by Pope Pius IX, December 8, 1854.
“For the honor of the Holy and Undivided Trinity, for the glory and adornment of the Virgin Mother of God, for the exaltation of the Catholic faith and the increase of the Christian religion, We declare, pronounce and define, by the authority of our Lord Jesus Christ, of the blessed Apostles Peter and Paul, and Our own, that the doctrine which holds that the most Blessed Virgin Mary, in the first instant of her conception, was, by singular grace and privilege of God, in view of the merits of Christ Jesus, the Saviour of the Human race, preserved free from every stain of original sin, has been revealed by God and is therefore to be firmly and constantly believed by all the faithful. Wherefore if any should presume, which God forbid, to think otherwise in their hearts than We have defined, let them know and be certain that they have been condemned by their own judgment, that they have made shipwreck in faith, that they have fallen away from the unity of the Church; and furthermore, that, should they dare to manifest in word or writing, or in any other external way, what they think in their hearts, they subject themselves by this very act to the penalties prescribed by law.”
Source: Catechism of the Council of Trent, Appendix I