What does Paul mean in Galatians 2:18 by rebuilding what he destroyed?

Paul means that after realizing the cross and faith in Christ's finished work had become preeminent, returning to the law and commanding converts to keep circumcision would rebuild what he had torn down. Doing so would make him a transgressor, because he would be restoring the law system he had abandoned.

Les teaches that Paul is reasoning from his own past. Paul had been a law-keeping Jew and a religious zealot, even killing people who accepted something contrary to that. When he realized all of that counted for nothing because the work of the cross had become preeminent, and that salvation is only through faith in the finished work of the cross, he tore down law-righteousness. Therefore, if he went back and put his converts under the law and commanded them to keep circumcision, he would be rebuilding what he had destroyed and would make himself a transgressor.

Galatians 2:18

For if I build again the things which I destroyed, I make myself a transgressor.

What did Paul say would happen if he put converts back under the law?

Paul said that if he told his converts he had been wrong and put them back under the law, he would be building again that which he had destroyed. The old economy had been done away with and nailed to the cross, so returning to it would rebuild what he had torn down.