15 What then was the blessing you enjoyed? For I bear you witness that, if possible, you would have plucked out your own eyes and given them to me.
16 Have I therefore become your enemy because I tell you the truth?
17 They zealously court you, but for no good; yes, they want to exclude you, that you may be zealous for them.
18 But it is good to be zealous in a good thing always, and not only when I am present with you.
blessing (v.15) — spiritual prosperity and happiness. They had declared themselves happy.
I tell you the truth (v.16) = I deal faithfully with you. The word, in Greek, includes integrity of conduct and speech.
The Galatians had been willing to do anything for Paul. He, throughout, dealt faithfully with them. But now, suddenly, they see him as an enemy, as one hostile to them.
they (v.17) — the Judaizers
zealously court (v.17) = take a warm interest in, although the word can also mean “jealous.”
for no good (v.17) — The methods and motives of the Judaizers were not intended to further the interests of the Galatians, in contrast to Paul’s behavior. Not honorable.
exclude (v. 17) — from assurance of salvation
that you may be zealous for them (v.17) — Once the Judaizers convinced the Galatians to doubt their salvation, the Galatians would be forced to seek the Judaizers to gain it.
Verse 18 — Paul says that it is good when the Galatians are warmly sought, if the motive is good, and not only when he is involved. It’s OK with him if others minister the truth to them (1 Corinthians 3:5; Philippians 1:15-18).
This is just the difference between proselytizing and evangelizing; in the one there is zeal for a creed, in the other for a person. The Judaizers paid court to the Galatians in order to attach them to a party; Paul took an interest in them in order that he might win them to, and preserve them for, Christ. — Galatians, by W.E. Vine, page 216