They zealously affect you. — “Zealously affect” is a single word in the Greek, and means “to show zeal towards,” “to court,” “to curry favour with,” “to canvass eagerly, so as to win over to their side.” The subject of this verse is the Judaising teachers.
They would exclude you. — They desire to separate you from the rest of the Gentile churches, and to make a sect by itself, in which they themselves may bear rule. All the other Gentile churches had accepted the freer teaching of St. Paul; the Judaising party wished to make of Galatia an isolated centre of Judaism. They did this with personal motives, “not well” — i.e., from honest and honourable motives — but with a view to secure their own ascendancy.
That ye might affect them. — The same word as “zealously affect” above and in the next verse. They expect to have all this zeal on their part returned to them in kind. With them it is the proselytizing zeal of the faction leader; from you they expect the deferential zeal of devoted followers.