Unleavened bread shall be eaten during those seven days. Nothing leavened may be found among you, nor shall leaven be found anywhere within your borders. — Exodus 13:7

Leaven in Scripture pictures sin and hypocrisy—a little spreads through the whole lump, which is why the Lord commands: let no leaven be seen in all your quarters. Search your heart and put away everything that offends him, not so you can be saved, but because Christ our passover has been sacrificed for us. We're called to be holy as he is holy, and that means dealing ruthlessly with the leaven of sin.