Why was the religious Jew condemned by the Law he thought he kept?
The Law condemned him because it exposed his old sin nature, which he would not have known except through the commandment. Although he thought he was keeping the Law, sin took occasion by the commandments and produced within him all manner of wrong desires.
Les explains from Romans 7 that the Law itself is not sin or wickedness; rather, it reveals sin. Paul says he would not have known his old sin nature except by the Law, and he would not have known lust except the Law said, "Thou shalt not." So the Law condemned this religious Jew who thought he was keeping it. His old sin nature took occasion by the commandments and wrought in him all manner of contraptions, getting his mind broiling. Les compares Romans 7 to difficult eighth-grade story problems that seem to boggle the mind before 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.