1 Corinthians 5:6-7

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.