Today’s solemnity in the Roman Catholic calendar, the Immaculate Conception of the Blessed Virgin Mary, is an interesting case of how doctrine develops and comes to be clarified very gradually over the course of church history.
By the early modern era, belief in Mary’s Immaculate Conception – that she had been conceived by human parents without acquiring any touch of original sin – had acquired a long and troubled pedigree. … As early as the twelfth century, the arguments about its validity were becoming passionate and even violent. Nor had the controversy subsided by the seventeenth century. The clashes over the Immaculate Conception remained so heated, in fact, that Popes Paul V and Gregory XV issued decrees in 1617 and again in 1622 that banned any public statements denying the truth of the doctrine. Although these decrees implicitly upheld the Immaculata, papal support was uneven through the century, especially during Ferdinand III’s reign. In 1642, for instance, Pope Urban VIII issued the bull Universa per orbem, which catalogued the universally required Catholic feasts and conspicuously omitted the Immaculate Conception; two years later he dealt a further blow to official acceptance of the doctrine with a decree forbidding any writer from using the adjective “immaculate” to modify “conception.” The controversy surrounding this doctrine persisted well beyond the seventeenth century; not until 1854 did it become dogma of the Catholic Church and receive an official liturgy. (Source: Sacred Music as Public Image for Holy Roman Emperor Ferdinand III by Andrew Weaver, p. 225, with footnote citations there.)
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In 1476 Pope Sixtus IV authorized those dioceses that wished to introduce a feast of Mary’s conception to do so, and he introduced it to the diocese of Rome in 1477. His bull Cum præexcelsa of 1477 referred to the feast as the Conception of Mary, without using the word “Immaculate.” He called Mary “immaculate,” but her conception he called “miraculous.”
In 1483 he condemned both those who called it mortally sinful and heretical to hold that the “glorious and immaculate mother of God was conceived without the stain of original sin” and those who called it mortally sinful and heretical to hold that “the glorious Virgin Mary was conceived with original sin”, since “up to this time there has been no decision made by the Roman Church and the Apostolic See.” This decree was reaffirmed by the Council of Trent.
Pope Pius V, while including the feast in the Tridentine Calendar, removed the adjective “Immaculate” and suppressed the existing special Mass for the feast, directing that the Mass for the Nativity of Mary (with the word “Nativity” replaced by “Conception”) be used instead.
On 6 December 1708, Pope Clement XI made the feast of the Conception of Mary, at that time still with the Nativity of Mary formula for the Mass, a Holy Day of Obligation. (Source: Wikipedia.)
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The decree of Pius IX in 1854, Ineffabilis Deus, emphasizes that the immaculate conception of Mary is only because of the grace of God and the merits of Jesus Christ the Savior:
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.