The Immaculate Conception
This is one of those feasts that is used by some people to say that Catholics give too much prominence to Our Lady. Their argument is that Mary, while she may truly be the mother of God, is still a person like the rest of us and that she had to wait to be redeemed by Jesus the same as everyone else.
However, the Catholic Church has maintained from very early on that Mary was conceived without the stain of Original Sin. The Church Fathers have held this position from very early on.
Origen, who was born around the year AD185, calls her ‘worthy of God, immaculate of the immaculate, most complete sanctity, perfect justice, neither deceived by the persuasion of the serpent, nor infected with his poisonous breathings.’
This is obviously a comparison to Eve and shows that Origen sees Mary as the second Eve.
Ambrose, around 400AD, says she is incorrupt, a virgin immune through grace from every stain of sin.
Finally, Maximus of Turin calls her a dwelling fit for Christ, not because of her habit of body, but because of original grace.
What the Church teaches is that Mary, like every other human being, had to be redeemed by Christ, but as a singular gift to her, Mary was ‘pre-redeemed,’ that is, what happened for us also happened for Mary, but as a special gift was given to her from the moment of her conception.
It was Pope Pius X who on Dec 8th, 1854 solemnly declared the Dogma of the Immaculate Conception, formalizing what most Catholics had held from the time of Jesus:
“We declare, pronounce and define that the doctrine which holds that the Blessed Virgin Mary, at the first instant of her conception, by a singular privilege and grace of the Omnipotent God, in virtue of the merits of Jesus Christ, the Saviour of mankind, was preserved immaculate from all stain of original sin, has been revealed by God, and therefore should firmly and constantly be believed by all the faithful.”
Dogma: Today, it is understood to be a truth appertaining to faith or morals, revealed by God, transmitted from the Apostles in the Scriptures or by tradition, and proposed by the Church for the acceptance of the faithful. It might be described briefly as a revealed truth defined by the Church — but private revelations do not constitute dogmas, and some theologians confine the word defined to doctrines solemnly defined by the pope or by a general council, while a revealed truth becomes a dogma even when proposed by the Church through her ordinary magisterium or teaching office. A dogma therefore implies a twofold relation: to Divine revelation and to the authoritative teaching of the Church. (from newadvent.org)