6 Your boasting is not good. Do you not know that a little leaven leavens the whole lump?
7 Cleanse out the old leaven that you may be a new lump, as you really are unleavened. For Christ, our Passover lamb, has been sacrificed.
8 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.
If you were to read this wish out reading the rest of the book of 1Corinthians who knows what you might think it would mean. This is why we read in context, that is with understanding of the rest of the book and in this case it is important to have read Exodus. In Exodus when the people of Israel were finally leaving Egypt, they were told by God via Moses, to have a Passover feast. During the night of this feast the angel of death passed by the doors of the Israelites because they followed the instructions of God to the letter.
The instruction we remember most is that of the painting of lamb’s blood on the doorway of the house. This is remembered because the blood of Jesus is now protects us. There was however another rule concerning the bread, the bread was not to have yeast in it. Leaven, I believe is a little leftover doe that you place in a new batch of doe in order to add the yeast.
For seven days no leaven is to be found in your houses. If anyone eats what is leavened, that person will be cut off from the congregation of Israel, whether he is a sojourner or a native of the land.
This was called the feast of the unleavened bread.
Now Paul in the above scripture compares the church to a new lump of doe and Christ to our Passover lamb. The old leaven is compared to malice and evil, as opposed to the unleavened bread of sincerity and truth.
So now for the application. As Christians we should not have malice and evil in our hearts but instead we should be sincere and truthful.