Suspecting the Opposition
Gal 4:17,18
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. It is fine to be zealous, provided the purpose is good, and to be so always and not just when I am with you.
People often become followers of a movement not because they are attracted to its beliefs, but because they are attracted to the zealousness of its charismatic leaders. Sometimes Christians are just too gullible and need to learn to "test the spirits to see whether they are from God, because many false prophets have gone out into the world." 1John 4:1 On the other hand there are also many Christians today who are too quick to judge and end up committing slander, bearing false witness against their brothers. This is particularly the case when it comes to making accusations concerning "motivations". Often when one person hates another for no good reason, they will make up all kinds of accusations concerning their opposition's motivation without dealing with the actual facts - namely what the person actually says and does. However, as we note in Galatians, Paul deals with his opposition much more objectively, revealing their doctrinal folly, and with regards to their motivation realize that Paul had much experience with such people as he records in chapter two, and so his critique of their motivations has a significant basis.
With respect to "zeal", it says in Prov 19:2 "It is not good to have zeal without knowledge, nor to be hasty and miss the way." But on the other hand it also says, "Do not let your heart envy sinners, but always be zealous for the fear of the LORD." Pr 23:17 Or if you'd like contrasting verses in the New Testament on zeal, there's:Rom 10:2 "For I can testify about them that they are zealous for God, but their zeal is not based on knowledge." And Rom 12:11 "Never be lacking in zeal, but keep your spiritual fervor, serving the Lord." So zeal is good and commendable as long as it is directed appropriately.