What happens if righteousness could come by the law?
If righteousness could come by the law, then Christ is dead in vain. That would frustrate the grace of God, making His death unnecessary. The context teaches believers are not under law but under grace, where God pours out undeserved favor and mercy on sinners because of His love at the cross.
Les explains that Paul refuses to frustrate the grace of God. In verse 21, Paul says, "I do not frustrate the grace of God, for if righteousness come by the law, by legalism, then Christ is dead in vain." Les emphasizes that believers today are not under the law but under grace. Grace is that attribute of God capable of pouring out undeserved favor and mercy on sinners, all because of the love poured out at the cross. That was love epitomized, as great an act of love as has ever been done, because He loved us as undeserving sinners. So if law-keeping could achieve righteousness, Christ's death would have been pointless.
I do not frustrate the grace of God: for if righteousness come by the law, then Christ is dead in vain.