Why don't Christians have to work and worry about whether they've done enough for God?
Christians do not have to work and worry about whether they have done enough because Christ did enough and accomplished all that was necessary. Their standing is by God's grace, not law-works. Grace is not a license to sin; the true believer receives different desires and does not want to live like the devil's crowd.
Les explains that the believer does not have to work and sweat and worry, hoping he has done enough, because Christ did enough and did all that was necessary. But grace is not licensed; it does not mean the believer can go out and live like the devil's crowd, mostly because he does not want to. The true believer would be as miserable in a honky tonk on Saturday night as a regular honky tonk customer would be miserable in church on Sunday morning. Two totally different lifestyles and desires make it work. This contrasts with Moses and the Law: Israel could not see the end of the Law system, and they are blinded to the fact that the whole system of Law has passed off the scene and is abolished, replaced by the revelation of the mysteries, the grace of God.