Charles Ellicott 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)

Your glorying is not good.—This possibly refers to some boasting about their spiritual state in the letter that had reached St. Paul from Corinth, to which this Epistle is, in part, a reply. (See 1 Corinthians 7:1.) As long as that one bad person is among you, it gives the whole community a bad character, just as leaven, though it may not have pervaded the entire lump, still means it is not the unleavened bread that was necessary for the Paschal Feast.

This Epistle, having been written shortly before Pentecost (1 Corinthians 16:8), was very likely composed around or soon after Easter; hence, the leaven and the Paschal Feast naturally suggest themselves as illustrations. The Apostle passes on rapidly from the mention of the leaven to the whole scene of the feast. Just as with the most minute and scrupulous care a Jew would remove every atom of leaven when the Paschal lamb was to be eaten, so, now that our Paschal Lamb has been slain, we must take care that no moral leaven remains in the sacred household of the Church while she keeps her perpetual feast of prayer and thanksgiving.