Yeast often symbolized sin (see Matthew 16:6, 12; Mark 8:15; Luke 12:1; Galatians 5:9). Jews ceremonially removed yeast from their homes before the annual Passover meal (Exodus 12:19; 13:7).
Get rid of the old leaven: If left unchecked, sin could spread throughout the church, just as yeast spreads through dough.