In 1 Corinthians 5:6–8 Paul uses the Passover image of leaven to stress the importance of church discipline.
Leaven (old, sour dough) was ordinarily worked into new dough in order to make it rise. During the seven-day feast after the Passover meal, the Jews were not allowed to have any leaven in the home (Exodus 12:18–20; Exodus 13:7). The absence of leaven pointed to the haste with which their forefathers had left Egypt (Exodus 12:33–34, Exodus 12:39).
Due to its powerful effect, leaven is often used in the Scriptures to represent a spiritual reality (see, e.g., Matthew 13:33; Mark 8:15; Galatians 5:9). In our verse, Paul uses it as a symbol for sin and defilement. The image of leaven also corresponds to the accusation that the Corinthians are puffed up
(1 Corinthians 5:2). Just as leaven makes bread rise, so undisciplined sin has led to arrogance in the church.
6 Your boasting is not good. Do you not know that a little leaven leavens the whole lump?