[referring to Mary]

I have quite a problem with the idea that Mary never committed a sin.
Was Mary fully human?  If it is possible for God to miraculously make 
a person free of original sin, and free of committing sin their whole
life, then what is the purpose of the Incarnation of Jesus?  Why can't
God just repeat the miracle done for Mary to make all the rest of us
sinless, without the need for repentance and salvation and all that?

I don't particularly object to the idea of the assumption, or the
perpetual virginity (both of which I regard as Catholic dogma about which
I will agree to disagree with my Catholic brothers and sisters in
Christ), and I even believe in the virgin birth of Jesus, but this
concept of Mary's sinlessness seems to me to be at odds with the
rest of Christian doctrine as I understand it.