Bill defines sin as a power source. Sin, power of sin, law of sin, and the principle of sin are all synonyms, according to Bill Gillham. If you look at most scriptures which use the word sin, sin is personified. By sin being personified, it can only be a person or power source rather than an act. Bill describes what happens to a lost persons mind, body, and spirit.
Most people think sinful nature is synonymous with flesh.
As we have our identity in Christ, logically and truthfully we can't have a sinful nature any longer. God doesn't reside in or with sin. I like how Bill says that we can't observe a our behavior and identify our nature based on it. Nor can we prove we are saved or determine we are saved by our performance. We can only determine our nature by birth.