Galatians 2:18 - acting as a charlatan

Galatians 2:18 (NLT)
Rather, I am a sinner if I rebuild the old system of law I already tore down.  

Galatians 2:18 (KJV)
For if I build again the things which I destroyed, I make myself a transgressor. 

The destroyer (14–21). Jesus destroyed the law by fulfilling it (Matt. 5:17–20). His death tore the temple veil (Luke 23:44–45) and removed the wall between Jews and Gentiles (Eph. 2:14–18). To go back to Moses is to rebuild what Jesus tore down and say that He did not really save us when we trusted Him.
Warren W. Wiersbe, With the Word Bible Commentary (Nashville: Thomas Nelson, 1991), Ga 2:1.

Peter had abandoned the whole legal system for faith in Christ. He had repudiated any difference between Jew and Gentile when it came to finding favor with God. Now, by refusing to eat with Gentiles, he is building up again what he once destroyed. In so doing, he proves himself to be a transgressor. Either he was wrong in leaving the law for Christ, or he is wrong now in leaving Christ for the law!

William MacDonald, Believer’s Bible Commentary: Old and New Testaments, ed. Arthur Farstad (Nashville: Thomas Nelson, 1995), 1880.

For if I build again. Return to the law. After Paul had preached that justification is only by faith in Christ plus nothing, it would be folly to seek righteousness by keeping the law. This would be building again that which he destroyed. Destroyed. This word means to demolish, dissolve, disunite, pull down. Paul destroyed the teaching that salvation was by works of merit by proclaiming that salvation was by grace through faith in Christ.

Edward E. Hindson and Woodrow Michael Kroll, eds., KJV Bible Commentary (Nashville: Thomas Nelson, 1994), 2381.

In Closing,

Galatians 2:18 (The Message Bible)
If I was “trying to be good,” I would be rebuilding the same old barn that I tore down. I would be acting as a charlatan.