“Why can’t you understand that I’m not talking about bread? So again I say, ‘Beware of the yeast of the Pharisees and Sadducees.’” Then at last they understood that he wasn’t speaking about the yeast in bread, but about the deceptive teaching of the Pharisees and Sadducees. [Matthew 16:11-12 (NLT)]
For a single-cell microorganism member of the fungus family, yeast is mighty powerful. When added to water and flour, it starts to grow and multiply as it ferments the sugars in the flour, releases carbon dioxide, and causes the dough to rise. Moreover, once added to something, yeast can’t be removed. When a small amount of old fermented dough called a starter or seor is kneaded into flour and water, it permeates the dough and makes it rise. Some of the newly leavened dough can be saved to become the starter for the next batch of bread and so on.
In the right conditions, yeast seems nearly immortal. San Francisco’s Boudin Bakery uses a sourdough starter originating in 1849. Scientists even revived yeast microbes from 4,500 years ago to make a loaf of bread! Indeed, the longevity, growth potential, and pervasiveness of yeast makes it a powerful substance.
The way yeast permeates and affects the dough with which it is mixed certainly makes it a good metaphor for the influence of one thing on another. Even though the Hebrew Scriptures never equated leavening with sin or evil, leaven and corruption had become almost synonymous with one another by the 1st century. Although Jesus used yeast’s power in a positive way as a metaphor for the growth and spread of the Messianic Kingdom, He also used yeast in a negative way; just as good can influence the world around it, so can bad.
At various times, Jesus used yeast as a metaphor for the power of evil to spread. He warned the disciples about the yeast of skepticism and unbelief found in the Pharisees, Sadducees, and Herod. In spite of the bountiful evidence of Jesus’ true identity seen in His miracles, the Pharisees and Sadducees demanded yet another “sign from heaven” to prove His authority. When Jesus appeared before Herod, the king wanted to see the proof of His miracles, as well. Not wanting His followers to be infected with such distrust or thinking of His miracles as entertainment for unbelievers, Jesus made this warning several times. Taking Him literally at first, the disciples thought Jesus was speaking of bread until they finally understood His meaning.
Along with the Pharisee’s skepticism, Jesus didn’t want his disciples influenced by their addition of the Talmud’s oral traditions to God’s final word in the Hebrew Bible or their hypocrisy in meticulously following the letter of the law while ignoring the most important commandment—that of loving God and their neighbor. Jesus also didn’t want His disciples affected by the Sadducees’ deceptive teachings. More concerned with their ritual purity than people’s welfare and politics than religion, the Sadducees denied things like angels, heaven, hell, and the resurrection while believing that people’s souls died with their bodies. As for Herod’s evil influence—Jesus didn’t want His disciples influenced by the actions of this immoral and self-indulgent man.
In letters to the Corinthians and the Galatians, the Apostle Paul also used yeast as a metaphor for the powerful influence of erroneous thinking and sinful behavior. When the Corinthian church ignored the flagrant immorality of one of its members, Paul warned them to remove him from the congregation lest such immorality spread through the entire congregation (as yeast does when added to fresh dough). In the same way, Paul warned the Galatians about the danger of accepting the perverted gospels of both the Judaizers (who insisted Gentiles had to adhere to Jewish laws like circumcision) and the Legalists (who preached a doctrine of salvation by works). Such false teaching was dangerous because, like yeast, it spreads out and affects everything with which it comes into contact.
Be it even a little false doctrine or immorality (whether sexual sin or things like abuse of power, financial fraud, deception, decadence, hate, hypocrisy, or gossip), when such evil is tolerated by the Church, it is like yeast. It’s evil works invisibly and will permeate and influence all that it touches. Just as a little leaven leavens the whole lump, a little sin can destroy the individual as well as the Church—the body of Christ. Let us beware!
Devotions of the Heart 