Galatians 4:16 "Have I now become your enemy by telling you the truth?"
Paul had brought the Gospel to the Galatians, and they had received it with joy. They had received Paul with joy as well. If they could have, they would have torn out their own eyes and given them to Paul.
Paul had left for a while, and humanness took over. Instead of clinging to the Gospel, the law crept back in. Their joy disappated as they forgot about that which is primary, and again focused on themselves and the law. And Paul doesn't hold back in letting them know the error of their way. So much so, that he fears it might make him their enemy.
But Paul does not let that fear hold him back. Because the truth is more important. It is almost as if he fears God more than men.
Verse 17 "Those people are zealous to win you over, but for no good. What they want is to alienate you from us, so that you may be zealous for them."
They wanted to turn it into a popularity contest. Paul knew all about being zealous, both for the right reasons and the wrong reasons.
If being zealous were all that mattered, Paul would have been saved as a Jew. But he wasn't. If fact, God delivered him from that state. Delivered him from knowing the law to knowing Christ. And Paul didn't want to go back at any cost, because he knew where he belonged. And he wanted the Galatians to know it too.