If, while we seek to be justified in Christ, it becomes evident that we ourselves are sinners, does that mean that Christ promotes sin? Absolutely not! If I rebuild what I destroyed, I prove that I am a lawbreaker.
This has always been a bit perplexing to me. Why would it become particularly obvious that we are sinners while we seek to be justified in Christ? And why would anyone think that if we slip up in trying to follow Christ, that this means Christ promotes sin? Also, why does rebuilding what I destroyed prove I'm a lawbreaker?
I was reading the passage rather quickly this time, so I still had in mind Paul's rebuke of Peter (verses 2:11-14) for encouraging Gentiles to live like Jews when he himself had been living like a Gentile... at least until some Jews belonging to the "circumcision group" came along and he began to shy away from the Gentiles.
It occurred to me that in verses 2:17-18 Paul is still talking about Peter, or rather, about the type of sin Peter had committed.
So if I might paraphrase the passage, I would say:
If, while we seek to be justified in Christ, it becomes evident that we ourselves are "sinning" by violating the old ceremonial law, such as the law of circumcision, does that mean that Christ promotes sin? Absolutely not! In fact, it is the other way around: If I rebuild the old, ineffective, ceremonial law that I destroyed - the way Peter has just been doing - that would really make me a lawbreaker.