… When the disciples reached the other side, they had forgotten to bring any bread. Jesus said to them, “Watch and beware of the leaven of the Pharisees and Sadducees.” … Then they understood that he did not tell them to beware of the leaven of bread, but of the teaching of the Pharisees and Sadducees.
– Matthew 16:1-12 (ESV)
The Pharisees and Sadducees are no longer with us, but their leaven is. You can’t see it, but you can see its effect.
Most bread today is leavened with plain yeast. But yeast in packets or jars was not available until relatively recently. Leavening in Bible times was accomplished by using a bit of dough from the previous batch to seed the next batch. We know this today as sour-dough. Chametz is the Hebrew word for bread made this way, which means sour, fermented.
Bread dough is inflated through the action of yeast and bacterial spores which produce carbon dioxide by converting the sugars in the flour to alcohols. Yeast is a living organism, and as it reproduces, it takes over the whole lump of dough (1 Corinthians 5:6 “a little leaven leaves the whole lump”).
In the Bible, leaven is first mentioned in connection with the Passover. God told the Hebrews to bake unleavened bread, because in the rush to leave Egypt, there would not be enough time to let the bread rise.
Once a year at Passover, the Jews got rid of the old sour-dough. God had commanded Israel to do this, but He gave no such command to the Gentiles. Today there are some famous sour-dough cultures that have been alive for tens or hundreds of years. The most famous is probably the one used to make San Francisco sour-dough bread. There are also well-known cultures from Naples that upscale pizzerias use.
In Matthew 16, Jesus was not condemning physical leaven. He was using leaven to represent teachings that ‘infect’ our thoughts – that start out small but eventually take over our whole mindset. The leaven of the Pharisees was one of hyper-legalism. It probably began hundreds of years before Christ with a few small commands intended to help people avoid sin, but it quickly expanded to the point where it virtually replaced the Law. Similarly, even today with our secular law, we see it is very easy to add more and more laws and regulations to the system, but very hard to remove them. (And the laws we add seem powerless to reduce crime.)
Natural leaven comes from the air. You can make leavened dough by mixing equal parts of flour and water, then leaving it out in the open for a while. The air is full of yeast, and eventually some will take residence in the dough, causing it to ferment. (For this reason, today’s Jewish dietary regulations consider dough to be leavened 18 minutes after it comes in contact with water.) But bread made with this dough might not taste good, depending on what variety of yeast takes up residence. Not all ‘wild’ yeast is the same. Some is good for baking and some is not.
Jesus compared the growth of the kingdom of heaven to leaven in Matthew 13:33 and Luke 13:21. But most of the time leaven is used in the Bible to symbolize sin. I think there’s an interesting analogy of yeast coming from the air. Satan is called the prince of the power of the air. In a spiritual sense, his bad “yeast” is all around us, and if we’re not careful, it will readily take over our hearts and minds. We must not let it do so. We need to remain unleavened…
Your boasting is not good. Do you not know that a little leaven leavens the whole lump? 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. 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.
– 1 Corinthians 5:6-8 (ESV)
Christians have a Passover to remember. When we share in the Lord’s Supper, we remember what it cost for Jesus Christ to free us from our sins. But we should also examine ourselves (1 Corinthians 11:28) for the leaven of malice and evil, hypocrisy and fleshly regulations, and so on. Let’s cast out the old leaven of sin, and not follow the Pharisee’s example of adding to what God has said.
Therefore let no one pass judgment on you in questions of food and drink, or with regard to a festival or a new moon or a Sabbath. These are a shadow of the things to come, but the substance belongs to Christ. Let no one disqualify you, insisting on asceticism and worship of angels, going on in detail about visions, puffed up without reason by his sensuous mind, and not holding fast to the Head, from whom the whole body, nourished and knit together through its joints and ligaments, grows with a growth that is from God. If with Christ you died to the elemental spirits of the world, why, as if you were still alive in the world, do you submit to regulations— “Do not handle, Do not taste, Do not touch” (referring to things that all perish as they are used)—according to human precepts and teachings? These have indeed an appearance of wisdom in promoting self-made religion and asceticism and severity to the body, but they are of no value in stopping the indulgence of the flesh.
– Colossians 2:16-23