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Jesus warns His disciples to "beware of the leaven of the Pharisees and the Sadducees." Seeing their puzzlement, He explains further. "Then they understood that He did not tell them to beware of the leaven of bread, but of the doctrine of the Pharisees and Sadducees" (verse 12). Both testaments use leaven as a symbol of sin because of what it does to a lump of bread dough. Once yeast enters the dough, it immediately begins to spread by breaking down in reaction to the dough's sugars and producing a gas that puffs the bread up.
Like leaven, when sin enters a person's life, it begins to corrupt and fill him with vanity. A person enslaved by habitual sin will have a difficult time growing in the grace and the knowledge of Jesus Christ because of sin's corrupting influence. Sin defiles and can permanently destroy relationships with God and man.
Throughout the year we hear frequent exhortations to produce fruit and grow in the grace and the knowledge of Jesus Christ. During Passover and the Days of Unleavened Bread, we give special emphasis to "getting the leaven out." These three actions are all parts of the same process. Though not technically the same, they are related closely enough to say they are simply different ways of describing the same process. "Getting the leaven out" is the most negative, "growing" is the most general, and "producing fruit" is the most specific. All three emphasize that a Christian should not stand still after entering the New Covenant. God expects him to take steps to ensure that these actions will occur in his life.
John W. Ritenbaugh
Five Teachings of Grace