Jamieson, Fausset and Brown on Galatians 2:18

18. Greek, "For if the things which I overthrew (by the faith of Christ), those very things I build up again (namely, legal righteousness, by subjecting myself to the law), I prove myself (literally, 'I commend myself') a transgressor." Instead of commending yourself as you sought to do (Ga 2:12, end), you merely commend yourself as a transgressor. The "I" is intended by Paul for Peter to take to himself, as it is his case, not Paul's own, that is described. A "transgressor" is another word for "sinner" (in Ga 2:17), for "sin is the transgression of the law." You, Peter, by now asserting the law to be obligatory, are proving yourself a "sinner," or "transgressor," in your having set it aside by living as the Gentiles, and with them. Thus you are debarred by transgression from justification by the law, and you debar yourself from justification by Christ, since in your theory He becomes a minister of sin.