1 Corinthians 5:6-8

6Your boasting [over the supposed spirituality of your church] is not good [indeed, it is vulgar and inappropriate]. Do you not know that [just] a little leaven ferments the whole batch [of dough, just as a little sin corrupts a person or an entire church]?

7Clean out the old leaven so that you may be a new batch, just as you are, still unleavened. For Christ our Passover Lamb has been sacrificed.

8Therefore, let us celebrate the feast, not with old leaven, nor with leaven of vice and malice and wickedness, but with the unleavened bread of sincerity and [untainted] truth.