Within the world of the bible, the symbol of leaven is used for a variety of purposes. It’s sometimes significant by its presence, and other times significant by its absence. It’s prohibited from use during the Passover festival, but permitted in offerings for thanksgiving. It’s used by both Jesus and Paul in the new testament as a symbol of growth and continuity.
Leaven and Passover
When God instituted the Passover festival in Exodus, the use of leaven was prohibited (Exodus 12:14-15). What exactly was leaven, and why was it prohibited? In ancient times, when bread was being made, they didn’t have the powdery yeast that we use today. Instead, a small piece of dough would be set aside and used as a starter for the next batch of bread to be baked. This small piece of dough was known as ‘leaven’. Sometimes, however, you could bake bread without the leaven and this was known as ‘unleavened bread’.
Why was leaven prohibited during the celebration of the Passover festival? It’s commonly suggested that perhaps leaven represents evil or corruption and for this reason should not be used during a holy festival. The problem with this line of argument is that leavened bread was permitted for use in thanksgiving offerings (Leviticus 7:13), especially during the festival of Firstfruits (Leviticus 23:17). If leaven symbolised evil, then leavened bread wouldn’t be permitted for use in any sacrificial offerings at all.
A better approach is to look at the context of the Passover regulations. God is intending to call his people out of Egypt in a sudden, dramatic way (Exodus 12:11). A piece of leaven takes time to work its way through the dough, and the Israelites need to be ready to leave at any moment. Leaven is associated with gradual growth and continuity with the past; each piece of leaven connecting each batch of bread with the one which came before. The exodus event represents a radical break with the past and so the Israelites are instructed to empty their houses of leaven to symbolise this urgency (Exodus 12:15, 19).
Growth and Continuity
There are other places in the Bible in which leaven is used to represent growth and continuity. One detail which needs accounting for is why leavened bread was permitted for thanksgiving offerings (Leviticus 2:11) but prohibited for grain offerings (Leviticus 7:13). The answer has to do with the place where the offering is made. Grain offerings were presented upon the altar and so were considered holy to God in a unique way, whereas thanksgiving offerings were simply ‘waved’ before the Lord. Even though God had given the land and its produce to the people of Israel as a good blessing, God and his altar still remained holy and set apart from everything that went on in the land.
In the gospels, Jesus uses the symbol of leaven in several passages. In the parable of the leaven, Jesus uses the symbol of leaven spreading through dough to talk about the kingdom of God spreading and filling the earth (Matthew 13:33). However, elsewhere he warns about “the leaven of the Pharisees and Sadducees” (Matthew 16:6), in reference to the way that their false teachings grow and spread like leaven through dough. The key idea is about leaven as something which grows and spreads – this can have either positive or negative connotations depending upon the context.
In 1 Corinthians 5, the apostle Paul uses the symbol of leaven to talk about habitual sins from the past which believers need to shed in order to embrace a new way of living in Christ (1 Corinthians 5:6-8). Within this passage, Paul brings together two symbolic aspects of leaven; the idea of growth and also the idea of continuity. Leaven is something which grows and spreads, hence why he warns them that “a little leaven leavens the whole lump” (v6). However, leaven is also something which establishes continuity with the past, which is why Paul calls the believers to “cleanse out the old leaven” and become “a new lump” (v7).
At the centre of Paul’s teaching is an analogy he draws between Christ and the Passover lamb. Since Christ has been sacrificed for us as our Passover lamb, just as the Israelites removed leaven from their houses, we should remove everything from our lives which dishonours God. This doesn’t mean that leaven in the original Passover narrative straightforwardly represented evil; it’s simply an analogy. But it nonetheless serves as a powerful reminder of what Christ has done and what that means for our lives today.
“For Christ, our Passover lamb, has been sacrificed. Let us therefore celebrate the festival, not with the old leaven, the leaven of malice and evil, but with the unleavened bread of sincerity and truth.” (1 Corinthians 5:7b-8)
 Within this verse, God instructs the Israelites to eat the Passover “with your belt fastened, your sandals on your feet, and your staff in your hand. You shall eat it in haste. It is the Lord’s Passover.” The people need to be ready to leave at any moment.
 At this moment in time, God also creates a new calendar for the people of Israel, establishing the Passover festival during the first month of their new year (Exodus 12:2-3). This break from the past will even involve a re-setting of the clocks!
 Of course, Israel cutting herself off from the past meant cutting herself off from Egypt and by implication the sins of Egypt. This remains a significant theme in the book of Exodus and could suggest an indirect association between leaven and corruption/evil in the Passover festival. But it’s not a straightforward connection.