Albert Barnes Commentary Galatians 5:9

"A little leaven leaveneth the whole lump." — Galatians 5:9 (ASV)

A little leaven, etc. This is evidently a proverbial expression. It is explained in the notes on 1 Corinthians 5:6.

Its meaning here is that embracing the errors they had adopted could be traced to some influence existing among themselves and acting like leaven. It may mean that a slight tendency to conform to rites and customs existed among them from the first, and that this tendency had now, like leaven, permeated the entire community. Alternatively, it may mean that the false teachers there could be compared to leaven, whose doctrines, though they were few in number, had permeated the community of Christians. Or, as many have supposed, it may mean that any conformity to the Jewish law was like leaven.

If they practised circumcision, it would not stop there. The tendency to conform to Jewish rites would spread from that until it would infect all the doctrines of religion, and they would fall into the observance of all the rites of the Jewish law. It seems to me that the second interpretation referred to above is the correct one; and that the apostle means to say that the influence which brought about this change was at first small and unimportant; that there might have been only a few teachers of that kind, and it might not have been deemed worthy of particular attention or alarm; but that the doctrines thus infused into the churches had spread like leaven, until the entire community had become affected.