What does Romans 7:21 mean?

Paul has been describing his inability to keep the law of Moses, to consistently obey all of God's Old Testament commands to Israel. Now he uses the word "law" to describe not the law of Moses but a universal principle of human behavior: Whenever we want to do what is right, evil is waiting nearby.

Paul's use of the word "evil" is striking. He is not describing his tendency to sin in the face of his good intentions as a bad habit or a personality disorder. He is describing sin as his desire to do evil, the opposite of good. Paul feels the desire to do what is right, and then he experiences the sin inside of him take over and choose to do evil instead.

Is Paul describing his experience as a non-Christian Jewish man, attempting to follow the law of Moses or as a Christian who has been freed from the power of sin but not from the desire to sin? Bible scholars disagree about that, though an analysis of Paul's Greek tenses here strongly suggests he's speaking of his present, personal condition. In either case, the principle that evil lies close at hand even when we set out to do good holds true. Christians are certainly capable of taking an unexpected left turn into sin even though we are no longer slaves to sin because Christ has set us free.