Why are all humans born with a sinful nature?
All humans inherit a sinful nature from Adam. The whole sin problem began with Adam's offense, and that condition has been passed down, so mankind's very nature is sinful. The law did not create sin; it revealed how sinful people already were.
Les explains that the law entered so the offense might abound, but this does not mean the law caused people to sin more. Rather, the law caused people to understand how sinful they were. He puts it this way: we are not sinners because we have broken the law; we break the law because we are sinners. The whole sin problem began with Adam, and we have inherited it, so the very nature of mankind is sinful. Even where sin abounded, God's grace did much more abound. Les also teaches that the three-lettered word 'sin' singular refers to the old Adam or the old sin nature, and it can be used synonymously with that.