Charles Ellicott Commentary Galatians 5:9

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

A little leaven . . .—A deeply significant expression, which leaves a good deal for the reader to supply. The proverb is true: a little leaven leavens the whole mass of dough. And so, in your case, the malcontents may be few, but they will soon ruin the whole Church. It seems decidedly more in accordance with the context to take the “little leaven” as referring rather to a few seceders than to a little bad doctrine.