A.T. Robertson Commentary Galatians 2:18

"For if I build up again those things which I destroyed, I prove myself a transgressor." — Galatians 2:18 (ASV)

A transgressor (παραβατην). Peter, by his shifts had contradicted himself helplessly as Paul shows by this condition. When he lived like a Gentile, he tore down the ceremonial law. When he lived like a Jew, he tore down salvation by grace.