In this passage, we see two kinds of zealousness. One that leads to good and one that does not.
We see here that the Galatians used to be zealous for Paul. Because of the gospel he had preached and the joy that had filled their hearts as a result, they were willing to do anything for him. (14-15)
Paul himself was zealous for the Galatians. You see it in every word, the hurt he felt because they had been deceived by the Judaizers. He said ,
My dear children, for whom I am again in the pains of childbirth until Christ is formed in you, how I wish I could be with you now and change my tone, because I am perplexed about you! (19-20).
He was so zealous for them that he told them the truth, even though they regarded him as an enemy for doing so (16).
But then there was the zealousness of the Judaizers. Paul said of them,
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. (17)
When he says, “They want to alienate you from us,” the “from us” is not actually in the text, and so the ESV puts it, “They want to shut you out.” The idea is that they were saying “You’re not really one of us. You’re not really a Christian. If you want to really be ‘in’ with Christ, you need to listen to us.”
But in doing so, they were locking up the Galatians under law all over again, and stealing the joy and blessedness God intended them to have.
In short, these people were zealous to win over these Galatians, but it was not for the Galatians’ ultimate good.
And so Paul tells them,
It is fine to be zealous, provided the purpose is good, and to be so always and not just when I am with you. (18)
Paul’s telling them, “Be zealous. That’s a good thing. When you were zealous for me, showing your love and caring for me because of the joy that was in you, that was good. I’m zealous for you too. I want to see Christ formed in you such that you become more and more like him.
“But these people are not looking out for your good. And by being zealous for them, you’re losing your joy and blessedness.”
So there’s two questions I think we need to ask. First, there may be some charismatic leaders around us, who are filled with zeal. But where are they leading you? Are they leading you closer to God? Or are they stealing your joy by what they are teaching you?
Are they teaching you truth, even though it’s painful to hear? Or are they leading you astray?
Second, what are you zealous for? Are you zealous for Christ and to know him? Are you zealous to see Christ formed in the lives of others that they may know his joy? Or is your zeal leading you in the wrong direction?
Zeal once led Paul to persecute the church. Zeal once led the Galatians back to a life of slavery under the law. Where is your zeal leading you?