John Calvin Commentary Galatians 5:4

"Ye are severed from Christ, ye would be justified by the law; ye are fallen away from grace." — Galatians 5:4 (ASV)

Christ has become of no effect to you. “If you seek any part of righteousness in the works of the law, Christ has no concern with you, and you are fallen from grace.” They were not so grossly mistaken as to believe that by the observance of the law alone they were justified, but attempted to mix Christ with the law. In any other point of view, Paul’s threats would have utterly failed to produce alarm. “What are you doing? You deprive yourselves of every advantage from Christ, and treat His grace as if it were of no value whatever.” We see then that the smallest part of justification cannot be attributed to the law without renouncing Christ and His grace.