Prove that from Scripture. No where does God in His Word EVER say Mary would be an exception to the rule. Mary herself said, "God MY SAVIOR" (Luke 1:47). If she were sinless and an 'exception' she would have NO need of a Savior. Scripture and more importantly Mary herself contradicts the RCC.
Secondly, this whole doctrine is in fact fairly new and not an apostolic tradition. It was first in 1547, at the council of Trent that the Catholic Church announced the sinlessness of Mary enabling her to avoid venial sins. In 1620 Pope Paul the 5th forbade anything contrary to the teaching of Mary's immaculate conception to be said publicly under threat of excommunication. In 1622 Pope Gregory the 15th forbade any contradictory statements of her immaculate conception to be made in private. And so began the official elevation of this servant woman the mother of the humanity of the Lord. When? Not until the 16th century!!!!!
Then Pope Pius IX in 1854 proclaimed, “Let all the children of the Catholic Church ... Proceed to worship, invoke, and pray to the most blessed Virgin Mary, mother of God, conceived without original sin"), but that the reason she never sinned at any time during her life was because she was unable to sin.” Then in 1950 Pius the 12th said, “Mary the immaculate perpetual virgin mother of God after the completion of her earthly life was assumed body and soul into the glory of heaven.”
So.....to quote words from you.....the church managed to survive over 1500 years before mandating a belief in the immaculate conception, and it wasn't until 1854 it became official dogma. In fact it was heresy to believe IN the immaculate conception prior to the Council of Trent:
“God alone is without sin. The only man who is without sin is Christ; for Christ is also God” (Tertullian, 215 A.D.; The Soul 41:3).
Augustine Bishop of Hippo “Whatever flesh of sin Jesus took, He took of the flesh of the sin of his mother. Jesus did not partake of sin, but took of his mother, which came under the judgment of sin.”
Augustine “ He, Christ alone, being made man but remaining God never had any sin, nor did he take of the flesh of sin. Though He took flesh of the sin of his mother.”
"If the Scriptures be duly considered, and the saying of the doctors ancient and modern, who have been most devoted to the glorious Virgin, it is plain from their words that she was conceived in sin,” (Cardinal Cajetan, De Loc TheoI. parts c. 2.)
“For he (Christ) alone was truly born holy” (Gregory the Great Bened. Edit. page 598).
" Pope Leo 1 (440 a.d.) “The Lord Jesus Christ alone among the sons of men was born immaculate”(sermon 24 in Nativ. Dom.)"
Pope innocent the third (1216 a.d.) “She (Eve) was produced without sin, but she brought forth in sin, she (Mary) was produced in sin, but she brought forth without sin.” ( De festo Assump., sermon 2) When the immaculate conception was first presented in the year 1140 it was opposed by Bernard of Clairvaux also Thomas Aquinas adamantly taught Mary was a sinner, and that any contrary view was heresy.
Third, In Luke 2:49-50 When Mary and Joseph after a day's journey found out that Jesus was missing from their company they went back to find Him. After two more days they found Him teaching in the temple teaching. Mary then asks Jesus why he did not leave with them, they looked high and low for him? His response is, why did you look all over for me? Did you not know that I must be about my fathers business (work)?”
Lk.2:50: “But they (both Mary and Joseph) did not understand the words he spoke to them.” Notice in both accounts Mary does not understand the things of God. Someone who is sinless would know God’s ways and not need a explanation! It is sin that corrupts ones understanding of spiritual things. What did they not understand? That Jesus would be about His Fathers business. In this account we see Mary equal with Joseph in not understanding.
In Luke 2:21-24 Mary brought a sacrifice of two turtledoves in accord to Jewish law in Leviticus chapter 12. One was for a burnt offering, the other was for a sin offering. This couldn’t have been for the child who was the Holy one, the sinless spotless lamb of God. This offering was for her own uncleanness as prescribed by Levitical Law. Mary's conformity to the law is an admission she was a sinner needing to be restored by the cleansing, only sinners need cleansing.
Fourth, yet another point of contention between the dogma of the immaculate conception and Scripture: If Mary is the woman of Revelation 12 as Roman Catholics say, it describes her with birth pains which according to the Bible is a judgment on sinners (Gen. 3:15-16). If Mary were immaculately conceived and sinless she would have had NO birth pangs.
Fifth, Mary did not need to BE sinless to conceive the sinless Son of God. Sin is transmitted through the seed of the man not the woman:
Romans 5:12 - Therefore, just as through one man sin entered into the world, and death through sin, and so death spread to all men, because all sinned-- (cf I Timothy 2:14)
Christ's sinlessness was not dependent on Mary being sinless, she didn’t need to be preserved from sin. Biblically the sin nature is passed on through the man. This is why it was not passed on to Jesus: He had no human Father to be part of His conception, but was conceived by the Holy Spirit.
The Bible teaches that sin is present at conception, as David the Psalmist stated, “in sin did my mother conceive me”, also in Job 14:4; “no one can make something clean out of something unclean.” This is why God gives man a new nature to rule over his sin nature.
Last, Mary had a human father, not Divine, (Luke 3:23) therefore she was in no way shape or form sinless or an exception. She herself would have had to have had a Divine father to be conceived without sin. Such teaching is heresy. Pope Gregory called it heresy, St. Augustine called it heresy, Jerome called it heresy, it was heresy in the 4th century when first introduced by Pelagious and it remains heresy in 2010 and you believe such lies at your own peril.