Regarding who is responsible for our sin, Calvin argues that Adam is not only the actual ancestor of mankind today, but also the 'federal head', in that he is representative of all of humanity, eg:
"Why feel any anxiety about the transmission of the soul, when we know that the qualities which Adam lost he received for us not less than for himself, that they were not gifts to a single man, but attributes of the whole human race? There is nothing absurd, therefore, in the view, that when he was divested, his nature was left naked and destitute that he having been defiled by sin, the pollution extends to all his seed."
This is particularly significant if you look at Genesis allegorically. Certianly though, there is no doubt that Calvin argues that original sin is passed to us through Adam.
Also, according to Calvin, to be in any way impure is to be sinful. Anything short of perfect righteousness is sin. There's no middle ground as many see it today, and I think this is where our confusion over total depravity/partial corruptness is coming from. Say for example, some person, who is not a Christian, gives money to charity. Pelagius or the Pope would say, "yeah, that was a good, righteous work of this man". Calvin would point out how he did not do it to honour God, and so it was sinful, and worthy of condemnation.
So when he argues how we are tainted, that doesn't just mean a sort of neutral agent burdened by sin - our imperfection is what makes us sin. Also, the fact that Jesus is said to be "made sin for us" on the cross is significant (a point Calvin makes a major deal about). It shows that the wrath of God is not against individual acts of sin, but against the sinful nature that produces them. Hence when Jesus comments on the law, he says how to think lustfully is adultery, to hate someone is murder etc. All this is designed to point us to the fact that sin is not just an act, but an inherent part of our nature.
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