"But purity cannot come from impurity."So how far back does this sinless line go? After all, Mary can't be sinless unless her mother was sinless. And her mother couldn't have been sinless unless Mary's grandmother was sinless. And so on and so on. So who was the first sinless woman?
This is a common question. Christ could not have needed redemption, for then he could not be the redeemer. Mary needed redemption. She was redeemed at conception, such that she was a pure (untainted by sin) vessel for Christ, and yet nevertheless in need of the salvation wrought by Christ. Both of those doctrines have to be preserved. There is no need for Mary's parents to be immaculately conceived, or Mary to be born of a virgin, because Mary did not need to receive sinless flesh at her conception in order to be Christ-bearer; she needed to be made sinless at no point later than her conception in order to be the Christ-bearer. She came into existence and was sanctified at the very same moment, such that she was never stained by stain (and thus could be a pure vessel for Christ) *and* yet also still needed the grace of Christ. Christ, however, never needed sanctification, and so could not receive sinful flesh at his conception. If Christ had needed sanctification (even at the moment of his conception), he could not have been the Savior. The Savior himself cannot be in need of salvation.
-A8