Exodus 12:8,9

Eat it not raw Nor half dressed; but roast with fire Not only because it might be sooner roasted than boiled, and they were in haste to be gone; but because it was thus the better type of him who endured the fierceness of divine wrath for us, Lamentations 1:13. Unleavened bread Partly to remind them of their hardships in Egypt, unleavened bread being more heavy and unsavoury; and partly to commemorate their hasty deliverance, which did not allow them time to leaven it, Exodus 12:39;

Deuteronomy 16:3. But as the original word for unleavened signifies pure, unmixed, uncorrupted, leaven being a kind of corruption, the use of unleavened bread, no doubt, was enjoined to show them the necessity of sincerity and uprightness: to which quality of leaven the apostle alludes, Galatians 5:2, and 1 Corinthians 5:8. With bitter herbs To remind them of their Egyptian bondage, which made their lives bitter to them.