ON this day, so dear to every Catholic heart, we celebrate, in the first place, the moment in which Almighty God showed Mary, through the distance of ages, to our first parents as the Virgin Mother of the divine Redeemer, the woman destined to crush the head of the serpent. And as by eternal decree she was miraculously exempt from all stain of original sin, and endowed with the richest treasures of grace and sanctity, it is meet that we should honor her glorious prerogatives by this special feast of the Immaculate Conception. We should join in spirit with the blessed in heaven, and rejoice with our dear Mother, not only for her own sake, but for ours, her children, who are partakers of her glory and happiness. Secondly, we are called upon to celebrate that ever-memorable day, the 8th of December, 1854, which raised the Immaculate Conception of Our Blessed Lady from a pious belief to the dignity of a dogma of the Infallible Church, causing universal joy among the faithful.
Reflection.— Let us repeat frequently these words applied by the Church to the Blessed Virgin: “Thou art all fair, Mary! and there is not a spot in thee” (Cant iv. 7).
Lives of the Saints, by Alban Butler, Benziger Bros. ed. 
Apostolic Constitution issued by Pope Pius IX on December 8, 1854.
Wherefore, in humility and fasting, we unceasingly offered our private prayers as well as the public prayers of the Church to God the Father through his Son, that he would deign to direct and strengthen our mind by the power of the Holy Spirit. In like manner did we implore the help of the entire heavenly host as we ardently invoked the Paraclete. Accordingly, by the inspiration of the Holy Spirit, 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 for the furtherance of the Catholic religion, by the authority of Jesus Christ our Lord, of the Blessed Apostles Peter and Paul, and by our own: “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 Savior 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.”
Hence, if anyone shall dare — which God forbid! — to think otherwise than as has been defined by us, let him know and understand that he is condemned by his own judgment; that he has suffered shipwreck in the faith; that he has separated from the unity of the Church; and that, furthermore, by his own action he incurs the penalties established by law if he should are to express in words or writing or by any other outward means the errors he think in his heart.