1 Corinthians 5: A Little Yeast Leavens the Whole Lump

Introduction

Who hasn't forgotten the yeast while making a braided loaf or a bread? The preacher admits it happened to him: the dough stayed flat, then it hardened. This message from the Gellertkirche in Basel, the third in a series titled "What's Wrong Here?", starts from that kitchen experience to talk about the dynamics of yeast in the Bible.

Yeast makes dough rise, and that is what we expect from it. But if you let it work too long, it turns the dough sour, makes it collapse, and ends up spoiling it. Three Bible passages serve as the common thread: Passover in Deuteronomy 16, Jesus' warning about the yeast of the Pharisees in Luke 12, and Paul's appeal to the Corinthians in 1 Corinthians 5.

Three questions structure the sermon: why no yeast at Passover? What is the yeast of hypocrisy? How does the yeast of sin work?

Outline of the Message

1. The Dynamics of Yeast: From Fermentation to Decay

Sourdough and baker's yeast do the same job, sourdough a little more slowly. Mixed with flour and water, they transform the sugar in the flour and the dough rises.
In Bible times, there was no dry yeast. People set aside a piece of the day's dough to start the next day's bread. This formed a chain of leaven that could go on endlessly.
Left a day or two longer, the leaven makes the dough too sour: it no longer holds, it collapses and goes limp. After a few days, or even two weeks, it decays, takes on a sharp taste of vinegar, alcohol, or even acetone, and mold appears.
"The effect we are looking for when we make bread is already the first stage of a process of decay: fermentation." This is why the Bible most often uses yeast as a negative image, for compromise, selfishness, or the desire to please people.
Sin works the same way: "At first, it looks so good. […] It smells good and promises satisfaction." In the moment, we don't see that it is starting to break down our values, our attitudes, and our actions from the inside.

2. Why No Yeast at Passover

Passover recalls the liberation from slavery in Egypt. It already prophetically announces the deliverance received through Jesus Christ. The blood of the lamb on the doorposts protected the houses: the angel of death passed over.
Israel received a clear instruction: do not take the yeast of Egypt along. For seven days, the people eat unleavened bread, to remember the haste of the departure, but also to mark a clean break. The unbroken chain of leaven is cut: it is the sign of a new beginning.
In current Jewish practice, Passover is also a call to repentance, purification, and reconciliation where there are disputes.
On the eve of the feast, after a week of cleaning, people go through the house with a candle looking for the last crumbs of leaven, in pockets, corners, and cracks. This recalls Proverbs 20:27: the spirit of man is the Lord's lamp, searching all his innermost parts.
For the preacher, this search is not done by our own light but by the light of God, just as the Paschal candle evokes the resurrection of Jesus. It is the prayer of Psalm 139: "Search me, God, and know my heart."

3. Searching Without Falling into Perfectionism

After the search, a formula is spoken: any leaven that I have not found, I declare null and worthless, may it be like the dust of the earth.
"We search carefully for the crumbs, but we are not required to find them all." What a person cannot see is declared void. This keeps self-examination from a meticulous perfectionism that leads nowhere.
David says it in Psalm 19: "Who can discern his errors? Forgive me from hidden faults."
An 18th-century Hasidic tradition notes that leavened bread and matzah are made of the same ingredients, water and flour. Only their state differs: one is swollen, the other stays flat. The swelling leaven speaks of pride, the matzah that stays flat speaks of humility.

4. The Yeast of Hypocrisy

Jesus speaks harshly to the scribes and Pharisees: they are like whitewashed tombs, beautiful outside, full of rot inside (Matthew 23:27-28).
In the Sermon on the Mount, he calls them hypocrites because they give and pray in public to be seen by people. Their worship is no longer about God but about their own reputation: what people think and say about them matters more than what God wants.
The preacher has recognized this temptation since his youth. People have often told him: "Wow, you are so impressive in your walk with Jesus," or "it's great, the way you make music." He confesses: "I am not immune to these things." An act meant to serve God can become a way to impress people.
In Luke 12, thousands of people crowd together so tightly that they trample one another. What an opportunity to proclaim the gospel, forgiveness, and a new beginning! Yet Jesus speaks first to his disciples: "Beware of the yeast of the Pharisees, which is hypocrisy."

5. How Hypocrisy Spreads

Hypocrisy is not only a danger for pastors and church leaders. It can quickly take hold of your heart when people admire your faithfulness at church, your service in the kitchen or at the café, or when they call you a "man of God" or a "woman of God."
Little by little, what others think of your commitment, and the number of "likes" under your social media post, become more important than receiving from God who you are in him.
"First it swells the ego, then it decays." The heart looks for confirmation, recognition, and identity from people rather than from God. It starts innocently, then it steals the inner orientation toward Jesus, peace, and joy in God.
Hypocrisy is contagious. It spreads to your friends, your prayer group, your home group. The preacher recalls youth groups where everyone ended up talking in a strange way, picking up expressions that were part of the group's code. This mechanism can work for the worse too.

6. The Yeast of Sin in the Church

In Corinth, a man is living with his father's wife, and the church tolerates it (1 Corinthians 5:1-2). Paul's main reproach is not aimed at the individual, but at the indifference and pride of the church, which saw its tolerance as a sign of freedom.
"Don't you know that a little yeast leavens the whole lump?" (1 Corinthians 5:6). Paul compares the whole church to a lump of dough: sin is not a simple private matter. Wrong behavior tolerated by the community ends up shaping it, until it becomes normal for everyone.
Paul calls for repentance, for a change of mind, and for clear action: remove the old yeast, name the evil, do not close your eyes.
The preacher sees two pitfalls. On one side, out of comfort, indifference, or a false idea of love and tolerance, we no longer see sin and we don't talk about it. On the other, a harshness without love excludes and confronts, leaving neither time nor room to enter a path of repentance.
Naming sin protects the whole church and serves the salvation of the person, so that they grow in their relationship with Jesus. The point is not to display a perfectly pure church: "Otherwise, I would not be here in front of you."

7. Christ, Our Passover: An Invitation to the Feast

Paul bases this appeal on what is already true: "you are unleavened. For indeed Christ, our Passover, has been sacrificed" (1 Corinthians 5:7). A theological line connects Israel's Passover to our time.
Christ took upon himself God's judgment on our rebellion and our own ways. That judgment passes over us because we are sheltered by his blood shed on the cross, by accepting his forgiveness.
"Repentance and purification are not the condition of salvation, they are our response to salvation." In Ezekiel 36, God gives a new heart and cleanses first, then the people remember their evil deeds and regret them. Christ loved us while we were still sinners (Romans 5:8), and God's kindness leads us to repentance (Romans 2:4).
Sanctification is nothing like a gloomy renunciation or self-denial: it is an invitation to a feast, like dressing up and putting on beautiful clothes. "Let's keep the feast, […] with the unleavened bread of sincerity and truth" (1 Corinthians 5:8).
On that day, the church celebrates communion, exceptionally but deliberately with unleavened bread, so that the image of the sermon is deeply impressed.

Key Takeaways

The yeast that makes dough rise is already the beginning of decay. Sin first seduces, then it spoils from the inside.
We search for leaven by the light of God, without falling into a perfectionism that leads nowhere: what we cannot see, God forgives.
Hypocrisy is serving God to impress people. It swells the ego, breaks down the relationship with God, and contaminates those around us.
In the church, tolerated sin shapes the whole community. It must be named, without indifference and without harshness.
Christ, our Passover, has been sacrificed: purification is not the condition of salvation but the joyful response to grace received.

Personal Application

What corners of your heart shelter old yeast: an old grudge, a refusal to forgive, a habit you justify, a media consumption that damages your values, bitterness, envy, half-truths?
Do you dare to pray "Search me, God," and make this examination in the light of the resurrection rather than with your own perfectionism?
In your service at church or your presence on social media, what matters most: the gaze of others or what God says about you?
Is there one thing you need to let go of with a clean break, like Israel leaving the yeast of Egypt behind?
As a church, what attitudes, what habits, what way of speaking have crept in among us? What do we tolerate simply because it is more comfortable?

Verses Cited

Frequently Asked Questions

Why does the Bible often present yeast as a negative image?

The preacher points out that the fermentation that makes dough rise is already the first stage of decay. Left too long, the yeast turns the dough sour, makes it collapse, and ends up spoiling it. Sin works in the same way: it begins by seducing and promising satisfaction, then it breaks down our values, our attitudes, and our actions from the inside.

What does Jesus mean by "the yeast of the Pharisees" in Luke 12?

Jesus means hypocrisy: a piety turned toward the gaze of others rather than toward God. He warns his disciples about it first, because this danger does not concern only religious leaders. Hypocrisy swells the ego, damages the relationship with God, and is passed on to those around us, like yeast that permeates the whole lump of dough.

Do you have to purify yourself to be saved, according to 1 Corinthians 5?

No. Paul writes to the Corinthians that they are already unleavened, because Christ, our Passover, has been sacrificed. Repentance and purification are not the condition of salvation, they are our response to the salvation received. As Romans 2:4 says, it is God's kindness that leads us to repentance.