“Your boasting is not good. Do you not know that a little leaven leavens the whole lump of dough? Clean out the old leaven so that you may be a new lump, just as you are in fact unleavened. For Christ our Passover also has been sacrificed. Therefore let us celebrate the feast, not with old leaven, nor with the leaven of malice and wickedness, but with the unleavened bread of sincerity and truth.” (1 Corinthians 5:6–8, NASB95)
Generally speaking, we think of leaven as a good thing. It is the leavening agent in bread that causes it to ferment and rise. Most commonly we use yeast in order to accomplish this. When it comes to breads like sourdough with its wild yeast starter they can be passed down from generation to generation as long as they are properly tended to. While there is no real record of the oldest starter, there is one that I read of that is over 122 years old which is kept in a refrigerator and re-fed every time its owner uses it. The small amount that she removes is then mixed with flour, sugar and water to make a dough for her bread. She then lets that dough sit for a couple of days, and then before using it she takes a portion of it and puts it back into her starter jar. In that time the amount that she took out, in order to make the dough, became so thoroughly permeated by the growing yeast that she was then able not only to make new bread but to re-feed and keep her old long-protected starter alive and well.
Looking back to the instructions given Israel for the Passover, they were to clean their homes and remove even the smallest portion of leaven. In the same way, leaven is used to refer to sin or in this case an individual engaged in persistent sin. Just as the leaven grows and ferments the entire loaf of bread causing it to rise, in verse 6 we see that their own harboring of this individual in their midst had led to their own swelling attitudes and the resulting boasting. The whole church was becoming infected, and Paul stepped in to take the drastic step of telling them to get him out of there.
The Corinthian believers were to become serious about the new life that they had been given resulting in their full and complete forgiveness of sin. This is who Christ made them to be, and this is how they were to responsively live. No longer were they slaves to sin. Its power over them had been broken at the cross. This is what we read when Paul wrote, “just as you are in fact unleavened.” This was a present condition statement that was to result in a life that reflected their current state.
Jesus Christ is the once and forever perfect Passover Lamb. No longer was an inferior sacrifice to be made. Jesus did it for us, and in Him we have been cleansed. We have become “unleavened.” We have been made righteous with His righteousness and set apart as His holy ones or saints (hagios). And, this truly is something to be celebrated, but not in the old ways that they celebrated and not doing those activities that they previously engaged in. We have not been set free in order to return to the muck. We have been set free to live free from that muck. We are made new creations in Christ to live purified of the old leaven as new unleavened loaves bursting at the edges of the pan of sincerity and truth.
“that, in reference to your former manner of life, you lay aside the old self, which is being corrupted in accordance with the lusts of deceit, and that you be renewed in the spirit of your mind, and put on the new self, which in the likeness of God has been created in righteousness and holiness of the truth.” (Ephesians 4:22–24, NASB95)