Romans 5:20 Now the law entered in, that sin might abound. And where sin abounded, grace did more abound.
So if Mary was “full of grace”……….
You cannot have grace where there is no sin.
If Mary were sinless, she could not experience either the grace of God nor the mercy of God. Both are given to undeserving sinners. If Mary never sinned, then she deserved what God gave her, which precludes grace and mercy.
Doesn’t the Word of God tells us that Jesus was tempted in all ways like we are? without a body and soul inherited from Adam how could He be tempted? Well, catholiciism claims He could not be tempted, of course contradicting the very Word of God! Jesus could have inherited a flesh able to be tempted only if Mary had such a flesh.