John Calvin 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. He condemns their glorying, not simply because they exalted themselves beyond what is lawful for humans, but because they delighted in their faults. He had previously stripped humankind of all glory, for he had shown that, as they have nothing of their own, whatever excellence they may have, they owe the entire praise of it to God alone (1 Corinthians 4:7). What he addresses here, however, is not that God is defrauded of His right when mortals arrogate to themselves the praise of their excellences, but that the Corinthians are guilty of utter folly in exalting themselves without any just cause. For they proudly gloried as if everything among them was in an ideal state, while in the meantime there was so much among them that was wicked and disgraceful.

Do you not know? So that they would not think that it was a matter of little or no importance that they gave encouragement to so great an evil, he shows the destructive tendency of indulgence and dissimulation in such a case. He uses a proverbial saying, by which he intimates that a whole multitude is infected by the contagion of a single individual. For this proverb has in this passage the same meaning as in those expressions of Juvenal: “A whole herd of swine falls down in the fields through disease in one of their number, and one discolored grape infects another.” I have said in this passage, because Paul, as we shall see, uses it elsewhere (Galatians 5:9) in another sense.