The Bible mentions leaven, or yeast, in several contexts. In some contexts, the reference to leaven is obviously literal; in other contexts, leaven takes on symbolic connotations.
Leaven causes dough to rise, but the process takes time. The Israelites, when God freed them from captivity in Egypt, had no time to spare, so, in their haste, they baked and ate flat (unleavened) bread for their journey: “With the dough the Israelites had brought from Egypt, they baked loaves of unleavened bread. The dough was without yeast because they had been driven out of Egypt and did not have time to prepare food for themselves” (Exodus 12:39).
To commemorate of His deliverance from Egypt, God instructed the Israelites to celebrate a week of feasting following the Passover Day (the 14th day of the 1st month on the Jewish calendar). This was called the “Feast of Unleavened Bread.” During that time the Israelites were commanded to remove all leaven from their houses and eat no bread that contained leaven (Exodus 12:15; 13:6–7).
Elsewhere in the Mosaic Law, leaven represents sin or corruption. The law forbade grain offerings made with leaven (Leviticus 2:11). In fact, no yeast was allowed to be burned on the altar in any sacrifice. The grain offering for Aaron and his sons (the priests) was also not to contain leaven and was to be eaten in a holy place (Leviticus 6:17).
Leaven is also mentioned in the New Testament. In Matthew 16:6–12, Jesus compared the false teaching of the Pharisees and Sadducees and Herodians to leaven. The Pharisees had come to Jesus to test Him (verse 1), but Jesus perceived their true intent and the state of their hearts. He later warned His disciples against being taken in by their teachings (verse 12), which He compared to leaven. A small portion of the “leaven” of falsehood can permeate a person’s heart and mind. In Luke 12:1 Jesus specifies that the leaven of the Pharisees is “hypocrisy.” Having a show of piety, without true holiness, is like leaven in that it gradually increases and spreads corruption, puffing up a person with vanity. Lies and hypocrisy can poison one’s whole character.
Paul warned the church at Corinth against tolerating sin in their midst, using leaven as a metaphor (1 Corinthians 5:1–8). There was a man in the church who was guilty of sexual immorality. Paul told them to remove the man from their fellowship because, like leaven, his influence would permeate the whole church. “Don’t you know that a little yeast leavens the whole batch of dough?” Paul asks (verse 6). Then he points them to the Passover and the Feast of Unleavened Bread: “Get rid of the old yeast, so that you may be a new unleavened batch—as you really are. For Christ, our Passover lamb, has been sacrificed” (verse 7).
In one of Jesus’ parables of the kingdom of heaven, He uses leaven in a different sense: “The kingdom of heaven is like yeast that a woman took and mixed into about sixty pounds of flour until it worked all through the dough” (Matthew 13:33). In this case, leaven is not used as a symbol of evil; rather, leaven is a symbol of the kingdom, which will gradually and secretly permeate society. Just as a woman uses the smallest bit of leaven in the dough, so the gospel starts with small beginnings. Just as the leaven quietly works its way through the whole batch, the gospel will have a profound impact on all sectors of society.