Gods' Law is perfect and unchanging, it is not nullified or changed, but our relationship to the Law has changed. Before the fall of man, we had no innate desire to eat the fruit of the tree of knowledge because we did not have a sin nature yet. After the Fall, the nature of man was corrupted, and our relationship to the Law became destructive. We know what sin is through the Law. In Christ, through a choice to be redeemed through his sacrifice on the cross, we can have a newness of identity and have a restored relationship to Gods' Law.