Galatians 2:18 (CSB) 18 If I rebuild those things that I tore down, I show myself to be a lawbreaker.

The system Paul tore down was his own Jewish traditions, according to the strictest sect of the Pharisees (Acts 26:5, Philippians 3:5-6) that would be the law based salvation. "The better I follow and obey the Law the greater chance I'll please God and get to heaven." However now Paul is building up the Christ based salvation. "I trust in Jesus as my Lord and Savior to get me to heaven." If Paul was to start preaching to the non Jews that they must follow the Law of God to get to heaven, then he is breaking the Law of God that Jesus is the only way to heaven. (John 3:16, John 14:6).