Your glorying is not good. Do you not know that a little leaven leavens the whole lump?
The Corinthian congregation had many problems, and the apostle Paul begins 1 Corinthians 5 by highlighting two of them.
First was a sin that even those outside the Church recognized as sexually immoral—“that a man has his father’s wife” (verse 1). The second was the response of the Corinthian congregation. They didn’t mourn or discipline the man, but instead were actually proud of themselves for their tolerance!
Paul used an analogy that was well-known to those who celebrated the biblical Feast of Unleavened Bread (Leviticus 23:6-8; Mark 14:1; Acts 12:3). Paul’s comments in 1 Corinthians 5:8 show that the Corinthian congregation was keeping or about to keep this important festival: “Therefore let us keep the feast, not with old leaven, nor with the leaven of malice and wickedness, but with the unleavened bread of sincerity and truth.”
During this festival, leaven, such as yeast, represents sin. One lesson is that, in the same way a little yeast can multiply to make a large batch of bread dough rise, so a sin in one person can spread to others. Also, a single sin in a person’s life can multiply into many sins.
Paul wanted the Corinthians to remove the sinner until he repented so that sin would not spread in the Church.
For more about the lessons of leaven, see “What Is Leaven and What Does It Picture?”