1 Corinthians 5:6-8

6 Your boasting is not good. Don’t you know that a little yeast leavens the whole batch of dough?
7 Get rid of the old yeast, so that you may be a new unleavened batch—as you really are. For Christ, our Passover lamb, has been sacrificed.
8 Therefore let us keep the Festival, not with the old bread leavened with malice and wickedness, but with the unleavened bread of sincerity and truth.