In the span of a heartbeat, the children of Israel were literally transferred out of the kingdom of slavery and into the kingdom of liberty, a community redeemed. This act of leaving Egypt with unleavened bread has led to one of the most colorful traditions of the Passover season. With great care they sweep and search and scrub their homes to remove every bit of leaven. Then just before Passover, bonfires are lit in empty lots and fields all over Israel to destroy any of the bread and crumbs that have been found. Rabbis point out that leaven puffs up bread the way pride does.
Flat bread speaks of humility and obedience.
Unleavened bread speaks of our readiness to put away from us the evil inclination that lives within all of us. It reminds us that freedom is not enough. God did not deliver His people from Egypt just for them to be free, but that they'd be free from the bondage of sin. He delivered His people to enjoy the liberty that is found by all who learn to live in humble dependence on the one true God. Purge out the old leaven, that you may be a new lump, since you truly are unleavened.