Paul was telling them to remove the evil, because the same thing could happen to them as to the bread when the yeast is put in it. When you put a little yeast in the bread dough and put the dough near the oven, of those ovens that are heated with firewood, by means of heat, the bread begins to swell. When the bread reaches a certain height, the baker or the person making the biscuits or bread quickly puts them in the oven. Why? To stop the yeast fermentation process; If the bread were not placed in the oven, the fermentation process would continue and the bread would continue to grow and the entire dough would spoil. And that, dear listener, is exactly what happens to evil in the church if one does not treat the evil that is within it. The situation would explode at any moment and destroy the effectiveness of the church. "A little leaven, said the apostle, ferments the whole dough." That is why he told them that they must be cleansed of the old leaven in order to be a new mass, without yeast.
In the Old Testament, after the Feast of the Passover, the Feast of Unleavened Bread immediately followed. Paul, then, said that Christ, the true Passover, has already been sacrificed for us. This was to be followed by lives that were free of yeast. But instead, this congregation in Corinth was allowing leaven, meaning evil, to enter their church. And they were precisely those who spoke of the crucifixion of Christ and of His death, and yet they allowed the leaven to penetrate the church. Then the apostle said to them here in verse 8 of this chapter 5 of the First Letter to the Corinthians:
"So let us celebrate the feast, not with the old leaven, nor with the leaven of malice and evil, but with unleavened bread, of sincerity and truth."
Paul was not talking about how a person was saved. He was commenting on the believer's life after he had been saved. Sincerity never saved anyone, but if you are a child of God, then you will have to be sincere. Today's world needs to see sincerity among believers, just as it needs truth among believers. And the apostle Paul said, "Let's have a little sincerity and truth in the Corinthian church." They were in the midst of tremendous immorality. They thought they could get away with it. But they were not sincere, and it seemed that everything was going well. They pretended to tell the truth and live that truth, when in fact they were lying.