Leaven is fermented bread dough that causes bread to rise. Leaven spreads. Leaven is an agent of growth. But there are different kinds of leaven. Bread bakers say that leaven actually takes on various characteristics of the climate and environment it is in. In this way, breads from different places in the world have slightly different characteristics and flavors based on the environment, altitude, temperature, and humidity.
In other words, leaven is basically a biblical way of talking about culture, or way of life, your environment, your greenhouse – what makes you grow. And therefore, there can be good leaven and bad leaven. When Israel left Egypt, God commanded them to get rid of their leaven because they were to leave the leaven of Egypt behind and start new loaves. The leaven of Egypt represented Egyptian culture, Egyptian gods, Egyptian ways of life. But this didn’t mean that Israel would not leaven their bread anymore, it meant that God was giving them a new start. God would bring them to Mt. Sinai and renew covenant with Israel and give them His law and instructions for worship and life in community, and this would become the new leaven of Israel, the new culture of Israel.
So here, Jesus warns His disciples and all of us to beware of the leaven of the Pharisees, which is hypocrisy. Paul applies this principle as well: “Therefore, let us keep the feast, not with old leaven, nor with the leaven of malice and wickedness, but with the unleavened bread of sincerity and truth” (1 Cor. 5:8). In other words, unleavened bread is another way of saying new leavened bread. We have a new leaven – it is the leaven of sincerity and truth. This is why it is perfectly fitting for our bread at the Lord’s Supper to be leavened or risen – this is the bread of the Kingdom that has been growing and leavening this world for 2000 years. But you must not come to this table with any of the old leaven. Leave malice and sin behind. This is the new leaven of sincerity and truth. Here, God tells the truth about your sin, He tells the truth about Jesus dying for it, and He calls you to leave it behind.
So come and keep the feast. Come and welcome to Jesus Christ.