1 Corinthians 5:6-8 (NIV)
Your boasting is not good. Don't you know that a little yeast works through the whole batch of dough? Get rid of the old yeast that you may be a new batch without yeast--as you really are. For Christ, our Passover lamb, has been sacrificed. Therefore let us keep the Festival, not with the old yeast, the yeast of malice and wickedness, but with bread without yeast, the bread of sincerity and truth.
There are few analogies or metaphors more consistent and prolific in the Bible than that of yeast or leaven being a type of sin. When you make bread, it just takes a little yeast to make a whole lump of dough rise nice and puffy. Think about that analogy. Sin grows like yeast, just a little can creep in, take hold, and pretty soon it can "corrupt" the whole person. Not only does this work in an individual, but it works on a national level too. The moral decline in our country today started with small, seemingly inconsequential things. To become clean and pure again, only a ruthless purging of sin will suffice. Any pet sins left unconfessed and unrepented of will only grow again.
What does yeast do to a lump of dough? Yeast is a living fungus which feeds on the bread and produces gas which puffs up the dough. The sin of pride works by puffing up too. How can you stop the growth of yeast? You have to put it into the oven and let the heat kill the yeast. God sometimes puts his own people into the fire of trials to stop the growth of sin in their lives.