Here:
Grace is presented (esp. in Paul) as the antithesis of sin.
To be full of such grace (simple logic) is to be without sin.
Mary was proclaimed by an angel as “full of grace” (Lk 1:28); therefore, she is without sin.
You will search in vain for the translation "full of grace" in Luke 1:28 in the King James Version, the New King James Version, the New Living Translation, the New International Version, the English Standard Version, the Christian Standard Bible, the New American Standard Bible, the New English Translation, the Revised Standard Version and the American Standard Version. Most of these say Mary is "highly favored".
Evidently the translators of these editions all must have been either a bunch of dummies, or a pack of anti-Catholic Protestant heretics to a man to get it so wrong, for so long.
The birth narratives of Jesus in Matthew and Luke were most likely composed to counter the calumny (to Christians) that Jesus was born of fornication (John 8:41). This became a bone of contention once Jesus' reputation had risen above mere "prophet" to deity. The solution to the charge of being a product of fornication was a miraculous birth to a virgin involving no human father at all.
But, of course, Mary then becomes the problem. She herself participated in sinful human nature, did she not, and therefore must have communicated it to her son, did she not?
So in Catholic theology a sinless Mary becomes necessary to stop the communication of sinful human nature to Jesus, based on tenous arguments such as above.
But then how did Mary escape the great chain of being? And if she did why was the birth of Jesus even necessary?
The whole thing quickly descends into more absurdity.
