Not Catholic but I have a question (not trying to derail at all). How does one reconcile teachings that say Mary was sinless with Scripture in Romans 3:23: "for all have sinned and fall short of the glory of God"?
The Immaculate Conception means that Mary, whose conception was brought about the normal way (a human mother and father), was by God's grace (and in light of Christ's merits) preserved from the stain of Original Sin.
The essence of original sin consists in the deprivation of sanctifying grace, and its stain is a corrupt nature.
Why would God preserve her from the stain of Original Sin at the moment of her conception? To prepare her for her role as Mother of Our Lord--He took on flesh and blood, her flesh and blood, and was born from her and was raised by her. She humbly and faithfully answered God's call when He sent the angel Gabriel to her:
"Hail, full of grace, the Lord is with you" (Luke 1:28).
The phrase "full of grace" is a translation of the Greek word
kecharitomene. It therefore expresses a characteristic quality of Mary.
The traditional translation, "full of grace," is better than the one found in many recent versions of the New Testament, which give something along the lines of "highly favored daughter." Mary was indeed a highly favored daughter of God, but the Greek implies more than that (and it never mentions the word for "daughter"). The grace given to Mary is at once permanent and of a unique kind.
Kecharitomene is a perfect passive participle of
charitoo, meaning "to fill or endow with grace."
Since this term is in the perfect tense, it indicates that Mary was graced in the past but with continuing effects in the present. So, the grace Mary enjoyed was not a result of the angel’s visit. In fact, Catholics hold, it extended over the whole of her life, from conception onward. She was in a state of sanctifying grace from the first moment of her existence.
Concerning the reference, "all have sinned" (Rom. 3:23) and Mary proclaiming that her "spirit rejoices in God my Savior" (Luke 1:47), I agree that Mary, too, required a Savior.
Like all other descendants of Adam, she was subject to the necessity of contracting original sin. But by a special intervention of God, undertaken at the instant she was conceived, she was preserved from the stain of original sin and its consequences. She was therefore
redeemed by the grace of Christ, but in a special way—by anticipation. The sanctifying grace that Christ won for redeemed Christians was applied to her at her conception.
But what about Romans 3:23, "all have sinned"? Have all people committed actual sins? Consider a child below the age of reason. By definition he can’t sin, since sinning requires the ability to reason and the ability to intend to sin (this also includes the severely mentally disabled).
This is indicated by Paul later in the letter to the Romans when he speaks of the time when Jacob and Esau were unborn babies as a time when they "had done nothing either good or bad" (Rom. 9:11).
We also know of another very prominent exception to the rule: Jesus (Heb. 4:15). So if Paul’s statement in Romans 3 includes an exception for the New Adam (Jesus), one may argue that an exception for the New Eve (Mary) can also be made.
The objection is sometimes also raised that if Mary were without sin, she would be equal to God. In the beginning, God created Adam, Eve, and the angels without sin, but none were equal to God. All of the holy angels in Heaven never sinned, and all souls in heaven are without sin.
This does not detract from the glory of God, but manifests it by the work He has done in sanctifying His creation.
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I remember thinking, I never learned that story in Sunday School 
