In Galatians 2:18, Paul writes, "If I rebuild what I destroyed, I prove that I am a lawbreaker." What is he referring to when he mentions rebuilding what was destroyed and how does that answer the objection that is raised in verse 17?
To understand what Paul is saying here in Galatians 2:17-18 it is helpful to read the context starting from Galatians 2:3-14.
However, to cut to the chase - Paul "destroyed" or "tore down" the idea that a person can be justified by the Abrahamic/Mosaic laws of circumcision/diet.
When Paul states, "...I prove that I am a lawbreaker.", he is saying that if he were to "put back into place" the idea that one can be saved through the works of the law (as Peter was hypocritically suggesting in Gal 2:11-14), then he (Paul) would be amiss.
I don't make a habit out of reading the New Living Translation, but its paraphrase on Gal 2:17-18 may be more helpful to you than the thoughts that I've shared already.
Gal 2:17-18 NLV - But suppose we seek to be made right with God through faith in Christ and then we are found guilty because we have abandoned the law. Would that mean Christ has led us into sin? Absolutely not! Rather, I am a sinner if I rebuild the old system of law I already tore down.
Keeping the law was never a means of salvation. It came 430 years after the Abrahamic Covenant (covenant of faith).
And this I say, that the covenant, that was confirmed before of God in Christ, the law, which was four hundred and thirty years after, cannot disannul, that it should make the promise of none effect. Galatians 3:17
For many reasons, I believe what Paul is referring to is sin...the works of the flesh.
Romans 7:13 states "...that sin by the commandment (law) might become exceeding sinful," means the law was given to us to show that we are incapable of keeping God's perfect standard for justification. The law was not given for our justification, but to show us our hopeless condition. Hence, we would look to Him who did not come to condemn the law or the prophets, but to fulfill (Matthew 5:17), so that by His merit we who belong to Him would be just as sinless. Humility for our brokenness, not pride in our "goodness" Luke 18:13-14, is what God intends for the law to reveal in us. If one exalts himself in his "law-keeping," he will be abased. Therefore someone who seeks to rebuild (live up to) the system of law for justification, which actually engenders pride not humility, will continue in the transgression described by Galatians 2:18, and ultimately be abased or rejected.
The law is not evil, but the perfect standard from God by which we should endeavor to live so that our lives will be better and glorify our Creator, but could never buy our forgiveness because we have all broken at least 1 commandment at least 1 time and are therefore without hope, without Christ (James 2:10).