“I do not make void the grace of God; for if righteousness is through the law, then Christ died in vain” (Galatians 2:21)
“I do not make void the grace of God,” says Paul in concluding the report of his speech to Peter;
“for if righteousness is through the law, then Christ died in vain.” The “for” here gives a reason for the use of the harsh word “make void”–” ‘make void,’ I say; for that is just the right word, since if Judaizers say, justification comes even in part through our obedience to the law, then Christ died in vain.”
This verse is the key verse of the Epistle to the Galatians; it expresses the central thought of the Epistle. The Judiazers attempted to supplement the saving work of Christ by the merit of their own obedience to the law. “That,” says Paul, “is impossible; Christ will do everything or nothing: earn your salvation if your obedience to the law is perfect, or else trust wholly to Christ completed work; you cannot do both; you cannot combined merit and grace; if justification even in slightest measure is through human merit, then Christ died in vain.”
J.Gresham Machen, Notes on Galatians. Edited by John H. Skilton, Solid Ground Classic Reprints, page 161.