My understanding of original sin is not that it is Adam's sin in our debit column, but rather that it refers to the radical change in human nature resulting from the fall. That corruption left us morally incapable of seeking God.
To which my good friend Craig commented:
I thought orthodox Calvinism taught exactly this - that Adam's sin is imputed to us in the same way that Christ's righteousness is.
I do not think that is correct. It is not in the same way that Christ's righteousness is imputed to us.
The WCF reads:
They being the root of all mankind, the guilt of this sin was imputed; and the same death in sin, and corrupted nature, conveyed to all their posterity descending from them by ordinary generation. (WCF VI-3)
My point was: original sin does not mean we are born essentially pure, only to be viewed by God as if we committed Adam’s sin. No, our situation is infinitely worse than that. We are born with a totally corrupted nature. As the WCF reads, there is an actual effect on us due to Adam's sin. We are not just regarded "as if" we had committed it. This effect renders us sinners in our own right from conception, for which we are justly condemned.
Although the WCF uses the word, this "imputation" is not the same as the two other, imputations we discuss—Christ’s righteousness to us and our sins to Him. Those are "as if" imputations. We are treated as if Christ's righteousness were ours, and He was treated as if our sin’s were his. The imputation of Adam’s sin was not in the same manner.