What does the Bible say about leaven?

In Scripture, leaven is often used to illustrate the corrupting influence of sin, particularly regarding pride and immorality, as seen in 1 Corinthians 5:6-8, where Paul warns that 'a little leaven leavens the whole lump.' Conversely, in Matthew 13:33, Jesus uses leaven positively to depict the growth and infiltration of the Kingdom of Heaven, showcasing how the gospel can start small but spread mightily. Thus, leaven embodies both the dangers of sin and the unstoppable nature of divine grace.