Jamieson, Fausset and Brown on Galatians 2:21

21. I do not frustrate the grace of God--I do not make it void, as thou, Peter, art doing by Judaizing.

for--justifying the strong expression "frustrate," or "make void."

is dead in vain--Greek, "Christ died needlessly," or "without just cause." Christ's having died, shows that the law has no power to justify us; for if the law can justify or make us righteous, the death of Christ is superfluous [Chrysostom].