1 Corinthians 5:7, Purge Out the Old Leaven
Barnes observed well, “The apostle here takes occasion, from the mention of leaven, to exhort the Corinthians to put away vice and sin. The figure is derived from the custom of the Jews in putting away leaven at the celebration of the passover. By the old leaven he means vice and sin; and also here the person who had committed the sin in their church. As the Jews, at the celebration of the passover, gave all diligence in removing leaven from their houses–searching every part of their dwellings with candles, that they might remove every particle of leavened bread from their habitations–so the apostle exhorts them to use all diligence to search out and remove all sin.”