The Immaculate Conception
Pope BI. Pius IX - 1854
God Ineffable — whose ways are mercy and truth, whose will is omnipotence itself, and whose wisdom “reaches from end to end mightily, and orders all things sweetly” — having foreseen from all eternity the lamentable wretchedness of the entire human race which would result from the sin of Adam, decreed, by a plan hidden from the centuries, to complete the first work of his goodness by a mystery yet more wondrously sublime through the Incarnation of the Word. This he decreed in order that man who, contrary to the plan of Divine Mercy had been led into sin by the cunning malice of Satan, should not perish; and in order that what had been lost in the first Adam would be gloriously restored in the Second Adam. From the very beginning, and before time began, the eternal Father chose and prepared for his only-begotten Son a Mother in whom the Son of God would become incarnate and from whom, in the blessed fullness of time, he would be born into this world. Above all creatures did God so loved her that truly in her was the Father well pleased with singular delight. Therefore, far above all the angels and all the saints so wondrously did God endow her with the abundance of all heavenly gifts poured from the treasury of his divinity that this mother, ever absolutely free of all stain of sin, all fair and perfect, would possess that fullness of holy innocence and sanctity than which, under God, one cannot even imagine anything greater, and which, outside of God, no mind can succeed in comprehending fully.
Supreme Reason for the Privilege: The Divine Maternity
And indeed it was wholly fitting that so wonderful a mother should be ever resplendent with the glory of most sublime holiness and so completely free from all taint of original sin that she would triumph utterly over the ancient serpent. To her did the Father will to give his only-begotten Son — the Son whom, equal to the Father and begotten by him, the Father loves from his heart — and to give this Son in such a way that he would be the one and the same common Son of God the Father and of the Blessed Virgin Mary. It was she whom the Son himself chose to make his Mother and it was from her that the Holy Spirit willed and brought it about that he should be conceived and born from whom he himself proceeds...
Mary Compared with Eve
Hence, it is the clear and unanimous opinion of the Fathers that the most glorious Virgin, for whom “he who is mighty has done great things,” was resplendent with such an abundance of heavenly gifts, with such a fullness of grace and with such innocence, that she is an unspeakable miracle of God — indeed, the crown of all miracles and truly the Mother of God; that she approaches as near to God himself as is possible for a created being; and that she is above all men and angels in glory. Hence, to demonstrate the original innocence and sanctity of the Mother of God, not only did they frequently compare her to Eve while yet a virgin, while yet innocence, while yet incorrupt, while not yet deceived by the deadly snares of the most treacherous serpent; but they have also exalted her above Eve with a wonderful variety of expressions. Eve listened to the serpent with lamentable consequences; she fell from original innocence and became his slave. The most Blessed Virgin, on the contrary, ever increased her original gift, and not only never lent an ear to the serpent, but by divinely given power she utterly destroyed the force and dominion of the evil one.
. . . Of a Super Eminent Sanctity
To these praises they have added very noble words. Speaking of the conception of the Virgin, they testified that nature yielded to grace and, unable to go on, stood trembling. The Virgin Mother of God would not be conceived by Anna before grace would bear its fruits; it was proper that she be conceived as the first-born, by whom “the first-born of every creature” would be conceived. They testified, too, that the flesh of the Virgin, although derived from Adam, did not contract the stains of Adam, and that on this account the most Blessed Virgin was the tabernacle created by God himself and formed by the Holy Spirit, truly a work in royal purple, adorned and woven with gold, which that new Beseleel  made. They affirmed that the same Virgin is, and is deservedly, the first and especial work of God, escaping the fiery arrows the the evil one; that she is beautiful by nature and entirely free from all stain; that at her Immaculate Conception she came into the world all radiant like the dawn. For it was certainly not fitting that this vessel of election should be wounded by the common injuries, since she, differing so much from the others, had only nature in common with them, not sin. In fact, it was quite fitting that, as the Only-Begotten has a Father in heaven, whom the Seraphim extol as thrice holy, so he should have a Mother on earth who would never be without the splendor of holiness.
This doctrine so filled the minds and souls of our ancestors in the faith that a singular and truly marvelous style of speech came into vogue among them. They have frequently addressed the Mother of God as immaculate, as immaculate in every respect; innocent, and verily most innocent; spotless, and entirely spotless; holy and removed from every stain of sin; all pure, all stainless, the very model of purity and innocence; more beautiful than beauty, more lovely than loveliness; more holy than holiness, singularly holy and most pure in soul and body; the one who surpassed all integrity and virginity; the only one who has become the dwelling place of all the graces of the most Holy Spirit. God alone excepted, Mary is more excellent than all, and by nature fair and beautiful, and more holy than the Cherubim and Seraphim. To praise her all the tongues of heaven and earth do not suffice...
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.
Given at St. Peter’s in Rome, the eighth day of December, 1854, in the eighth year of our pontificate.