Preserved from sin from the very first moment of her conception

The Eastern Churches have always celebrated Mary's original purity with a feast of “The Conception of the Holy Mother of God”, i.e. Mary's conception in the womb of her mother Saint Anne. 

The Latins gradually adopted it from the 10th century onwards, but St. Bernard, St. Bonaventure and St. Thomas Aquinas were still reluctant to admit this “Immaculate Conception”. Saint John Duns Scotus was the first to champion it and have it accepted by the Sorbonne in Paris. The Popes intervened many times over the centuries to silence this quarrel, until Pius IX defined it as a dogma of faith in 1854: 

“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, on the first day of Creation, when Adam and Eve came forth from the hands of the Creator, the mother of His Son was there, a tiny human cell endowed with an all-holy soul. She thus became the glory of our sinful nature.

Papal Encyclical

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