Those people are zealous for you, but not in a good way. Instead, they want to isolate you from us, so that you may be zealous for them.
17. They are jealous of you. He comes at length to the false apostles, and does more by silence to make them odious, than if he had given their names; for we usually abstain from naming those whose very names produce in us dislike and aversion. He mentions the immoderate ambition of those men, and warns the Galatians not to be led astray by their appearance of zeal.
The apostle is still carrying on the same design as in the foregoing verse, which was, to convince the Galatians of their sin and folly in departing from the truth of the gospel: having just before been expostulating with them about the change of their behaviour towards him who endeavoured to establish them in it, he here gives them the character of those false teachers...
Commenting on Galatians 4:17-18
But it is good to be zealously affected,.... A zealous affection when right is very commendable, as the instances of Phinehas, Elijah, John the Baptist, and our Lord Jesus Christ show, and a contrary spirit is very disagreeable. But then it must be expressed in a good thing; in a good cause, for God, and the things of Christ; for the Gospel, and the ordinances...