Galatians 2:21

I do not make void the grace of God: for if righteousness is through the law, then Christ died for nought. [I do not, Peter, in following my course, make void the grace of God which gave us Christ. But your course does this very thing, for if a man can be righteous and obtain justification under the law, then the death of Christ is superfluous. Paul's rebuke to Peter is not only a complete climactic justification of his claims as an apostle, but forms also a most fitting introduction, both in matter and spirit, to his immediately following rebuke of the Galatians, who were, like Peter, returning to the law.]