John Gill Commentary Galatians 5:9

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

A little leaven leavens the whole lump.
This is a proverbial expression pretty much in use with the Jews; see (1 Corinthians 5:6). It respects either persons or things.

The expression is in answer to, or prevents an objection that might be made, or something that might be said, in favour of these churches: that their case was not so bad, or the danger they were in so great, as was represented by the apostle. This objection might argue that since they were only a few persons that propagated this notion, and but few that received it, at least thoroughly gave into it; and that, if it was an error, it was but a small one, and only regarded a single ritual, or a few rituals of the law.

To this the apostle replies, by supposing, but not granting this to be the case (since they were pretty generally declined, and the error was not a slight one), that as a little sour leaven influences and ferments a large mass, or lump of dough, and makes it of the same nature with it, so a small error in doctrine, as it may be thought to be, increases to more ungodliness, and eats as does a canker. And though a few hands may be first concerned in propagating it, and but few be infected with it, yet these may soon spread the contagion through the whole society.

Therefore, errors and false teachers should be nipped in the bud, and stopped in their beginnings, how inconsiderable soever they and their tenets may be judged to be.