Your glorying - Your boasting; or confidence in your present condition, as if you were eminent in purity and piety.
Is not good - Is not well, proper, right. Boasting is never good; but it is especially wrong when, as here, there is an existing evil that is likely to corrupt the whole church. When people are disposed to boast, they should at once make the inquiry whether there is not some sin indulged in, on account of which they should be humbled and subdued. If all individual Christians, and all Christian churches, and all people of every rank and condition, would look at things as they are, they would never find occasion for boasting. It is only when we are blind to the realities of the ease, and overlook our faults, that we are disposed to boast. The reason why this was improper in Corinth, Paul states - that any sin would tend to corrupt the whole church, and that therefore they ought not to boast until that was removed.
A little leaven ... - A small quantity of leaven or yeast will pervade the entire mass of flour, or dough, and diffuse itself through it all. This is evidently a proverbial saying. It occurs also in Galatians 5:9. Compare the note at Matthew 13:33. A similar figure occurs also in the Greek classic writers - By leaven the Hebrews metaphorically understood whatever had the power of corrupting, whether doctrine, or example, or anything else. See the note at Matthew 16:6. The sense here is plain. A single sin indulged in, or allowed in the church, would act like leaven - it would pervade and corrupt the whole church, unless it was removed. On this ground, and for this reason, discipline should be administered, and the corrupt member should be removed.