Petty Theft - Stealing Back Grace

Just a bit of Scripture below, and this observation to 75% of the Christians I know:  What do you not understand about "you are not under law, but under grace"????? (Rom 6:14).

Galatians 2:16-21 ¹⁶ Nevertheless, knowing that a man is not justified by the works of the Law but through faith in Christ Jesus, even we have believed in Christ Jesus, so that we may be justified by faith in Christ and not by the works of the Law; since by the works of the Law no flesh will be justified. ¹⁷ "But if, while seeking to be justified in Christ, we ourselves have also been found sinners, is Christ then a minister of sin? May it never be! ¹⁸ "For if I rebuild what I have once destroyed, I prove myself to be a transgressor. ¹⁹ "For through the Law I died to the Law, so that I might live to God. ²⁰ "I have been crucified with Christ; and it is no longer I who live, but Christ lives in me; and the life which I now live in the flesh I live by faith in the Son of God, who loved me and gave Himself up for me. ²¹ "I do not nullify the grace of God, for if righteousness comes through the Law, then Christ died needlessly."