What does Paul mean when he uses the singular 'sin' in Romans?
Paul uses the singular 'sin' in Romans to denote the old Adam, the old sin nature. It is the inherited condition from Adam, not merely individual lawbreaking. People are not sinners because they broke the law; they break the law because they are sinners by nature.
Les explains that in Romans 5:20 Paul writes that the law entered so the offense might abound. The law did not cause people to sin more; it made them understand how sinful they already were. The sin problem began with Adam and was inherited, so mankind's very nature is sinful. The law could only show man how sinful he was. Les calls the singular word 'sin' the old Adam, or the old sin nature, and says it can be used synonymously without doing violence to Scripture. God remains greater, so where sin abounded, grace did much more abound.