A.T. Robertson Commentary 1 Corinthians 5:6

"Your glorying is not good. Know ye not that a little leaven leaveneth the whole lump?" — 1 Corinthians 5:6 (ASV)

Not good (ου καλον). Not beautiful, not seemly, in view of this plague spot, this cancer on the church. They needed a surgical operation at once instead of boasting and pride (puffed up). Καυχημα is the thing gloried in.

A little leaven leaveneth the whole lump (μικρα ζυμη ολον το φυραμα ζυμο). This proverb occurs verbatim in Ga 5:9. Ζυμη (leaven) is a late word from ζεω, to boil, as is ζυμοω, to leaven. The contraction is regular (-οει=ο) for the third person singular present indicative. See the parables of Jesus for the pervasive power of leaven (Matthew 13:33). Some of the members may have argued that one such case did not affect the church as a whole, a specious excuse for negligence that Paul here answers. The emphasis is on the "little" (μικρα, note position). Lump (φυραμα from φυραω, to mix, late word, in the papyri mixing a medical prescription) is a substance mixed with water and kneaded like dough. Compare the pervasive power of germs of disease in the body as they spread through the body.