Galatians 2:18 “If I rebuild what I destroyed, I prove that I am a lawbreaker.”
Verse 18 closes the loop started with the previous verse, verse 17. Paul’s arguments about how some might conclude that relying on faith for salvation rather than a moral code leads to immoral living is designed to refute the Judiazers at the foundation of their own argument. The tone here is different than in Romans 7, or even later in Galatians 5, where the heresy or deception is with those who already recognize salvation is by faith and the grace of God, and used the argument of faith vs. law as an excuse to justify immoral behavior. The focus here is different. The Judiazers apparently recognized that Jesus was the Messiah and that His death was required for our redemption. But they also insisted on the continued need to follow the law. Part of the reasoning behind this was that the Law was designed to keep us on the right path – away from evil – to set God’s people apart from the wickedness of the world. A foundational point in the Judiazer’s argument would be “without the law, and relying on faith rather than the rules of the law, will lead to immoral behavior, for the law defines moral behavior.” It’s the inverse of the argument Paul refutes in Romans 7, which was “I’m free of the law in Christ, therefore I can do whatever I want.” Paul refutes both of these arguments here, first in v. 17 (by being a little sarcastic) he's pointing out that to argue that reliance on faith for justification will lead to immoral behavior leads to the ridiculous conclusion that Jesus promotes sin, or, at least faith in Jesus will promote sin. It also admits to the concept that we are all sinners by nature, and that we are bound to fail and make mistakes as we go.
This latter concept is continued here in verse 18. “If I rebuild what I destroyed.” If we go back to relying on the law for justification, the result is the same, the conclusion does not change – “I prove that I am a lawbreaker.” In other words, I am a sinner by nature. All we prove by trying to rely on the law is that we will invariably break the law. It seems hopeless, then, doesn’t it? Taken by themselves, verses 17 and 18 seem to say – “We seek justification by faith, and we discover we are still sinners. We follow the law, we prove we are still sinners.” Will we ever change? Where is the hope for freedom from sin? The rest of Galatians 2 is a snapshot of the theology of how Christ changes us from sinner to sanctified – as indeed the rest of the book of Galatians does as well. But Paul starts his presentation in the negative, I believe, to set up the concept that reliance on the law for salvation fails, in order to refute the Judiazers outright. Indeed, the dramatic emphasis of Galatians 2:21 is the ultimate refutation of this argument.