God uses everyday items, the familiar and mundane, to teach many truths. Throughout the Old Testament we see how He gave the Hebrews symbols and metaphors to make something small mean much. For example, leaven or hometz was forbidden from most sacrifices, and it was explicitly forbidden at Passover time. Leaven is usually seen as a symbol of sin. Notice the parallels.
By nature leaven is sour or bitter. Similarly, sin tends to embitter people and sour life. Sin often shows itself in bitter words: Hide me from the secret plots of the wicked, from the rebellion of the workers of iniquity, who sharpen their tongue like a sword, and bend their bows to shoot their arrows—bitter words" (Psalms 64:2,3).
Leaven pervades that which is unleavened. No one ever made a barrel of rotten apples to be good by putting in a fresh apple. But anyone can make a barrel of good apples become rotten by putting in a bad apple. The same yeast fungus that makes bread rise also makes apples rotten.
Sin also has a contaminating or pervasive effect. This is what the Apostle Paul meant when he said, "…Do you not know that a little leaven leavens the whole lump?" (1 Corinthians 5:6). Leaven is an agent that puffs up its host. Yet as bread rises, it has no more substance or weight than unleavened bread; it just takes up more space.
Even so, sin causes us to become puffed up: self-righteous, selfcentered and self-pitying when faced with the disappointments of life. Pride and vanity add nothing to our substance, but cause us to imagine ourselves as more than what we are. The Bible warns that God will flatten those who are puffed up with sin: "For the day of the LORD of hosts shall be upon everything proud and lofty, upon everything lifted up—and it shall be brought low" (Isaiah 2:12).
The Bible not only gives us parallels between leaven and sin, but provides specific examples. Jesus warned against the "leaven of the Pharisees and of the Sadducees" (Matthew 16:11) and in verse 12, He identified it as their doctrine. Luke 12:1 specifically identifies the leaven of the Pharisees as "hypocrisy." In 1 Corinthians 5:8, Paul identifies leaven as "malice and wickedness" which is juxtaposed to "sincerity and truth."
To understand the symbolism more fully, we need to look at the process as well as the characteristics of leaven. In ancient times the method of baking leavened bread was similar to the sourdough method still used to make San Francisco-style bread. There was an original culture of leaven, which passed from one loaf of bread to another; a piece of the leavened dough would be torn from the pre-baked loaf, wrapped in a damp cloth and kept in a cool place to leaven the next batch of bread. Again, a lump would be torn from the next batch and set aside for the following loaf. Even so our sin has passed from generation to generation. Passover symbollically breaks the cycle of this generational sin. "Therefore purge out the old leaven, that you may be a new lump, since you truly are unleavened. For indeed Christ, our Passover, was sacrificed for us" (1 Corinthians 5:7).