Confrontational Ministry
Gal 2:11 When Peter came to Antioch, I opposed him to his face, because he was clearly in the wrong.
Sometimes you just have to opposed fellow Christians to their face, even if they be the apostle Peter (or your institutional church leader). This is part of appropriate Christian ministry. What checks and balances did Peter have in his life to maintain his humility and to help keep him free from the hypocrisy of which institutional leaders are particularly prone? Who dared correct Peter when he screwed up? Paul dared, knowing full well that he was risking Peter's endorsement of his ministry.
Paul brings up this incident
1. to show the clarity and confidence of his conviction on this matter\
2. and to provide an example to for the Galatians to follow - namely to oppose the circumcision to their face.
Doesn't matter who they are. If people are clearly in the wrong on this matter, they need to be confronted. (And yes, expect to be rebuked - if not crucified - in return.)