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.
According to early church interpretation it's the law or the works of the law.1
1: Tertullian, adv. Marc., 5,3,8; Eusebius of Emesa, in Gal.; Ephrem, in Gal.; Augustinus, Exp. Gal. 16,7-10; Ambrosiaster; Theodoret of Cyrus