What does Paul mean by 'sin' in Romans?
In Romans, Paul uses 'sin' to mean the old sin nature. The Law is not sin; it reveals and condemns the old sin nature, showing that good, nice, church people are sinners because all are guilty of breaking God's perfect Law.
Les explains that in Romans the best explanation for 'sin' is the old sin nature. In Romans 7:7, Paul asks whether the Law is sin and answers, 'Banish the thought.' No, Paul says, I had not known sin—my old sin nature—except by the Law. He would not have known lust except the Law said, 'Thou shalt not do it.' So the Law condemned this religious Jew who thought he was keeping it. But sin, his old sin nature, taking occasion by the commandments, wrought in him all manner of contraptions and got his mind broiling. Romans 7 is like an eighth-grade story problem: it boggles the mind until the answer comes out.
What shall we say then? Is the law sin? God forbid. Nay, I had not known sin, but by the law: for I had not known lust, except the law had said, Thou shalt not covet.