So the question has been posed: if Mary was sinless for her entire life, that is, entirely perfect, then wouldn't she have been able to atone for our sins by dying on the cross? She, too, would have been the "perfect sacrifice".
A Protestant presented her argument this way: Had Mary been sinless throughout her life, she could have bore the sin of the world and would have been the perfect sacrifice and Jesus Christ could have stayed where He was and He wouldn't have had to humble Himself into deepest humiliation and anguish to die on the cross for our sins. If Mary had been sinless she would have been the one to fulfill the Law by keeping it perfectly.
Quick response: Jesus' death on the cross atoned for the sins of the world not because he was sinless, but because he was GOD.
Only an infinite being can offer atonement infinitely for the sins of humankind.