Why was the Law powerless to help people?
The Law was powerless because, though perfect, it was weak and had no ability to enable obedience. It sat in cold stone, severe and condemning, showing what was wrong but never supplying the inner power or desire to change. Only the indwelling Holy Spirit gives that desire and strength to live pleasing to God.
Les explains that the Law was not imperfect; the Ten Commandments were perfect. Yet it was powerless because it could only sit there in cold stone. It was severe, condemned, and did nothing to help a person keep it. It was like a mirror: after working outside, you look and the mirror shows what is wrong and needs cleaning, but it cannot reach out and clean you. It is helpless and weak. By contrast, the indwelling Holy Spirit is warm, loving, embracing, and gives believers the desire to live pleasing in God's sight, taking away the desire to rebel. That work is the Holy Spirit's, not ours.