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 love 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.
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.
The Catholic Church, directed by the Holy Spirit of God, is the pillar and base of truth and has ever held as divinely revealed and as contained in the deposit of heavenly revelation this doctrine concerning the original innocence of the august Virgin—a doctrine which is so perfectly in harmony with her wonderful sanctity and preeminent dignity as Mother of God—and thus has never ceased to explain, to teach and to foster this doctrine age after age in many ways and by solemn acts. From this very doctrine, flourishing and wondrously propagated in the Catholic world through the efforts and zeal of the bishops, was made very clear by the Church when she did not hesitate to present for the public devotion and veneration of the faithful the Feast of the Conception of the Blessed Virgin. By this most significant fact, the Church made it clear indeed that the conception of Mary is to be venerated as something extraordinary, wonderful, eminently holy, and different from the conception of all other human beings—for the Church celebrates only the feast days of the saints.
And hence the very words with which the Sacred Scriptures speak of Uncreated Wisdom and set forth his eternal origin, the Church, both in its ecclesiastical offices and in its Liturgy, has been wont to apply likewise to the origin of the Blessed Virgin, inasmuch as God, by one and the same decree, had established the origin of Mary and the Incarnation of Divine Wisdom.
From the Apostolic Constitution "Ineffabilis Deus" of Pope Pius IX