21 I do not set aside the grace of God. For if righteousness comes through the law, Christ died for nothing. -Galatians 2:21 BSB
The word “grace” in Greek means God’s kindness or gift or favor to mankind (i.e., Jesus paying for the sins of the world even though men didn’t do anything to earn such a sacrifice from the Lord).
Paul makes it clear that if you set aside God’s gift (Christ’s righteousness applied to a person by faith), you are saying indirectly that you can live a better life than He did.
What must be understood is that no one can keep the law and accept the grace of God at the same time. A person cannot pay for a gift that is given to him. Either he pays for it and gets it by his means or receives it as a gift acknowledging that he could have never afforded it. Salvation is given to the humble while the prideful try to keep the law and earn salvation by their merit, rejecting Jesus Christ as his Lord and Savior.