William Burkitt on Galatians 2:21

The apostle concludes the chapter with a double argument, to prove the Christian's justification by faith, without the works of the law.

1. Were it otherwise, we should frustrate and make void the grace of God: For if justification be by works, it can no more be by grace; according to the apostle, Rom 6:6.

2. Christ's death had been in vain, without any necessary cause or reason at all, if the justification of a sinner could have been obtained by his own works.

Where note, that as well works done after faith and conversion, as those done before it, are excluded from being the meritorious cause of our justification, either in whole or in part; because the joining of works with faith, in the matter of our justification, is a total exclusion of God's free grace, and a loud proclaiming, that Christ died in vain: If righteousness come by the law, I frustrate the grace of God, and Christ is dead in vain.