Do you want to know the devil's most common way, among Christian denominations, of undermining the Gospel, and preventing essential reforms? It is the exhortation not to be divisive. Maintain "unity" (so-called). Don't make people upset.
As Solomon might say, there's a time to make peace, and there's a time to be confrontational. Paul tells the tale of a time that the apostle Peter became intimidated by some Jewish-Christian visitors from Jerusalem. Until that time, Peter fellow-shipped and ate with Gentile-Christian brothers freely.
But when Peter saw these new arrivals, whoever they were, he started avoiding his Gentile friends. Peter's bad example in turn started influencing other Jewish believers to commit the same sin. You can imagine how hurt and angry they felt. Peter's attitude and example was putting them back into second-hand status, based on race.
Paul never says whether the Jerusalem visitors gave Peter the stink-eye. For all we know, they might have had completely innocent attitudes! Maybe Peter assumed certain things. This incident appears to have happened before the Jerusalem council (Acts 15).
Peter wasn't being a racist. Peter was attacking the Gospel itself, through his cowardice. Because if one needed to become a Jew to be acceptable to God, that meant one needed to be a law-keeper to be acceptable to God. Because, after Genesis 17, to be a Jew and to be a law-keeper were the same thing. So Peter wasn't just being rude to his friends. He was attacking the Gospel! Though I'm sure Peter didn't think that's was what he was doing.
What should Paul had done? Kept his mouth shut? That's what some evangelicals would counsel. Talk to Peter privately? No, Peter did this publicly. Paul dealt with this severe problem on the spot, because it needed that.
If we love the unity of the body of Christ, then we stop the perversion of the Gospel no matter who is doing it. It doesn't matter if it's your best friend, your mentor, the founder of your association, your father, or your son. Ihe minister who does not oppose heresy is divisive.
I once said to a dear friend of mine -- a friend I have known since 1975, whom I led to Christ, who is like a brother to me, whose best man I was, and who is a pastor in the same denomination as mine -- that if he ever deviates from the Gospel, I will lead the process to have him removed. And I expect him to take the same stance toward me. If I were to start denying the Trinity, or denying the deity of God the Son, or denying that salvation is by grace alone through faith alone through Christ alone, I expect my real friends to love God, love the Gospel, and love me by getting right up into my face about it. Otherwise, are they really my friends?