This is a great question. Actually, its several great questions. How can the Catholic Church say that Mary never sinned, when Paul says that all have sinned? Well, its simple, really: In the Bible, all doesnt always mean all individuals. Sometimes it means some of all types. For example, when Jesus said, And I, if I be lifted up from the earth, will draw all men to Myself (John 12:32, NAS), He did not mean that He would draw each and every man, woman, and child to Himself. He meant that He would draw some people from every tribe and language and people and nation (Rev. 5:9). In other words, it wasnt just the Jews that would benefit, it was all people, all nations.
This is the sense in which Paul meant Romans 3:23. Paul spends the first two chapters of Romans demonstrating that the Jews and Gentiles are both equally guilty before God. In Romans 3:9, he writes, We have already made the charge that Jews and Gentiles alike are all under sin. Then he writes, There is no difference [between Jews and Gentiles], for all have sinned and fall short of the glory of God (Rom. 3:22-23). That is, all nations are under sin, the Jews included.
On an individual level, however, there can be exceptions without violating the meaning of this verse. Obviously, Jesus Himself is an exception. He did not sin. Neither do young children, who, because of their immaturity, are incapable of making moral choices, and thus, are incapable of committing actual sins. Therefore, Mary could also be an exception, if God chose to make her one. Of course, could be is a long way from is, but Im only trying to demonstrate here that the doctrine of Marys sinlessness does not contradict the meaning of Romans 3:23 (that both Jews and Gentiles as a group, are sinners). Statements like all have sinned, while generally true, are not to be understood to mean that there can be no exceptions. For example, we know that as a general rule, the wages of sin is death (Rom. 6:23), but we also know that both Enoch (Gen. 5:24) and Elijah (2 Kings 2:11), though they were sinners, never died.
Now, its certainly true that in the normal course of things, every baby is conceived in a state of Original Sin. Had God not intervened, this would also have been true of Mary. Likewise, it is also true that every person who lives long enough to be capable of moral decision-making will commit actual sins. Again, had God not intervened, this would also have been true of Mary. But the Catholic Church points out that there was nothing normal about the job Mary was given to do. Her role was different and more important than any other persons in human history: to bear, give birth to, feed, clothe, protect, raise, and train the Son of God.
Catholics simply believe that God gave Mary gifts that were appropriate to her role as Jesuss mother. It was not intrinsically necessary that she be preserved from sin, but it made her a more fitting vessel to bear the Son of God. And by the way, her preservation from sin was a gift, based solely on the merits of her Son. She, being human, was as much in need of a savior as the rest of us. But her role in salvation history was more difficult and more important than any other persons. (Who among us would dare to say that his own role in history was more important than bearing and raising the Son of God?) Therefore, by Gods grace, she was saved more perfectly, and more comprehensively, than the rest of us. In essence, we are pulled out of the pit of sin; she was prevented from falling into it in the first place. So it is entirely appropriate for Mary to call God her savior, for He saved her in a most spectacular way.
If we look back in history, we find that Christians have known about Marys sinless condition from the very beginning. There are hints of it as far back as the second century, when it was common to compare her to Eve, who was also undefiled. It is explicitly taught somewhat later:
You alone and your mother are more beautiful than any others, for there is no blemish in you nor any stains upon your mother. Who of my children can compare in beauty to these?1
Come, then, and search out your sheep, not through your servants or hired men, but do it yourself. Lift me up bodily and in the flesh, which is fallen in Adam. Lift me up not from Sara but from Mary, a virgin not only undefiled but a virgin whom grace had made inviolate, free of every stain of sin.2
Having excepted the holy virgin Mary, concerning whom, on account of the honor of the Lord, I wish to have absolutely no question when treating of sins for how do we know what abundance of grace for the total overcoming of sin was conferred upon her, who merited to conceive and bear him in whom there was no sin? so, I say, with the exception of the Virgin, if we could have gathered together all those holy men and women, when they were living here, and had asked them whether they were without sin, what do we suppose would have been their answer?3
Its actually rather ironic that modern Protestants object to the Immaculate Conception and subsequent sinlessness of Mary, because, like the early Christians, Martin Luther, the founder of Protestantism, was a firm believer in it. He wrote, It is a sweet and pious belief that the infusion of Marys soul was effected without original sin; so that in the very infusion of her soul she was also purified from original sin and adorned with Gods gifts, receiving a pure soul infused by God; thus from the first moment she began to live she was free from all sin.4
1 Ephraim the Syrian, Nisibene Hymns, 27:8, A.D. 361.
2 Ambrose of Milan, Commentary on Psalm 118, 22:30, A.D. 387.
3 Augustine, Nature and Grace, 36:42, A.D. 415.
4 Martin Luther, Sermon, On the Day of the Conception of the Mother of God, 1527.
It appears you fellas are better informed than any who have ever lived.