What would it mean if righteousness came by the law?
If righteousness came by the law, then Christ died in vain. His death would be unnecessary because people could be made right through legalism. This would frustrate the grace of God and replace undeserved favor with law-keeping, contradicting the cross.
Les explains that in verse 21 Paul refuses to frustrate the grace of God. Paul says, "if righteousness come by the law, by legalism, then Christ is dead in vain." Les points out that believers today are not under the law but under grace. Grace is God's attribute of pouring out undeserved favor and mercy on sinners, all because of the love poured out at the cross. If law-keeping could produce righteousness, then Christ's death would have been pointless, and grace would be set aside.
I do not frustrate the grace of God: for if righteousness come by the law, then Christ is dead in vain.