Leaven, to my understanding, is a rising agent in baking. So, why does the Bible constantly use it as an example for us? I think it's because we understand these kinds of examples: it doesn't take much leaven to make bread. And along that same line of thought, if you DON'T want leavened bread, it doesn't take much to ruin your bake if there is any left over from earlier in your cooking. We will apply the same logic to Christianity today on our discussion.