The Corinthians were proud of their church while serious sin went unchecked in it. Paul says plainly, "Your glorying is not good," and then asks a question they already knew the answer to. Barnes explains that their glorying was "confidence in your present condition as if you were eminent in purity," and that boasting "is especially wrong when, as here, there is an existing evil." Gill notes that the saying about leaven was a proverb "much used by the Jews" and "common in the mouths of all." Paul uses something everyone knew from the kitchen to show them what they were ignoring: sin left alone does not stay small.

Paul answers with a command: "Purge out therefore the old leaven, that ye may be a new lump, as ye are unleavened" (1 Corinthians 5:7). He gives the reason: "For even Christ our passover is sacrificed for us." Then he draws the conclusion: "Therefore let us keep the feast, not with old leaven, neither with the leaven of malice and wickedness; but with the unleavened bread of sincerity and truth" (1 Corinthians 5:8). He ends the chapter with the practical step: "put away from among yourselves that wicked person" (1 Corinthians 5:13).

Leaven was a piece of old fermented dough used to make new dough rise. A small piece worked its way through the whole batch. At Passover, Israel was commanded to "put away leaven out of your houses" for seven days (Exodus 12:15, 19). Barnes notes how carefully the Jews searched their houses to remove every bit of leavened bread, and Gill mentions the search made "by the light of a lamp." The Greek word for leaven is zyme, and "glorying" is kauchema, boasting. Paul uses the same proverb in Galatians 5:9.

Paul's question is not only a warning. It points to the gospel. "Christ our passover is sacrificed for us" (1 Corinthians 5:7). Barnes draws the comparison: "we, when our paschal lamb is slain, should put away all sin from our hearts and from our churches." Christ's sacrifice has already made the church "unleavened," and so they must live like what they are. Matthew Henry says, "The sacrifice of our Redeemer is the strongest argument with a gracious heart for purity and sincerity."

Barnes applies the proverb to the church: "A single sin indulged in, or allowed in the church, would act like leaven." But even here Paul's aim is restoration. The man was to be dealt with "that the spirit may be saved in the day of the Lord Jesus" (1 Corinthians 5:5). Even the discipline Paul commands aims at the good of the one disciplined, not only at the purity of the church.