If you read this verse in context, it’s clear that Paul is in some sort of dispute with the Galatians. Though the story is a little out of order (this isn’t a narrative text, it’s a letter), what seems to have happened is this:
- Paul arrived in town because of an illness, was taken care of and shown great hospitality, and used this as an opportunity to preach to them. Many converted and followed his preaching. (Galatians 4:13)
- Sometime after Paul’s departure, many of those who he had converted began reverting to their pre-conversion ways. We gather this from the questions Paul asks in verse 9 and the rebukes he offers in 10-11:
Now, however, that you have come to know God, or rather to be known by God, how can you turn back again to the weak and beggarly elemental spirits? How can you want to be enslaved to them again? You are observing special days, and months, and seasons, and years. I am afraid that my work for you may have been wasted. (NRSV translation)
- In verse 15, Paul first suggests that there may be trouble in paradise between him and the Galatians:
What has become of the goodwill you felt? (NRSV)
This statement seems to imply that this is not the first interaction between Paul and the Galatians since he left town and they changed course. In other words, Paul has spoken to them about this already, probably lodging some of the same rebukes he’s giving here, and they haven’t taken it well.
Which gets us to verse 16:
Have I now become your enemy by telling you the truth? (NRSV)
So when Paul says he’s become their enemy, it’s clear he’s referring to the current animosity between him and the Galatians. I read “by telling you the truth” to be a reference to whatever prior interactions he has had with them in which he has given the same rebukes he’s giving here (rebukes which presumably they have responded to with hostility). In a way, this is just a reflection of human nature: nobody really likes being criticized. Paul is defending the criticism he’s given by making an impassioned argument all throughout this letter for the gospel of “freedom” from the bondage of sin that he preached to them. But it’s also not that surprising that when he tells people they are behaving like slaves (in a culture in which slavery was very much a live practice and slaves were the lowest rung of the totem pole, often prisoners from defeated “barbarian” tribes) that they aren’t taking it all that well.