1 Corinthians 5:7

The old leaven] Not (or, at any rate, not only) the unchaste sinner, but the spirit in the Church which is indifferent to the sin. Christ our Passover] The mention of leaven, which was associated with the Passover, causes the Apostle to think of that institution; and leads him to speak of Christ in allegorical fashion as the Christian Church's Passover. As Christ has been sacrificed the days of the spiritual feast of unleavened bread have begun; and consequently every vestige of impurity and malice and sin must be rigorously excluded.