In Galatians 2:4, the 'false brethren' who spy out liberty are the ones whose teaching Paul would be rebuilding by yielding — verse 18 warns of that transgression.

In Galatians 2:12-16, Peter's withdrawal from Gentiles is the rebuilding of the law-based divide Paul had destroyed — verse 18 warns against such regression.

In Galatians 2:21, Paul equates building again the law with frustrating grace — verse 18's 'transgressor' is one who nullifies grace.

In Galatians 4:9-12, Paul warns against returning to 'weak and beggarly elements' — the same relapse as rebuilding what was destroyed in verse 18.

In Galatians 5:11, Paul's hypothetical preaching of circumcision would be rebuilding the law — parallel to the 'building again' that makes a transgressor.