Galatians 5:9

Leaven became a type of moral and spiritual corruption in virtue of the fermentation it engenders. A very small lump might readily form a centre of widespread corruption; so stringent precautions were adopted in Jewish households for the removal of every particle before the days of unleavened bread. Hence the origin of the proverb quoted here and in 1 Corinthians 5:6. It is clear that the taint of heresy had not yet spread widely through the Galatian Churches: it was more its insidious nature than its actual extent that alarmed the Apostle.