Answer: This is a good question, and we will need to dig into the text of the OT to come up with an answer. Now in the OT, outside a reference to unleavened bread in Genesis 19:3 in which Lot prepared a feast for his two angelic visitors that included unleavened bread, we don’t see unleavened bread until Exodus 12 and the institution of the Passover celebration. The story is very familiar to Christians. Right before God inflicts Egypt with the tenth plague, the death of the firstborn, God commands to Moses to mark this great occasion in which the Lord delivered the people of Israel from slavery and bondage in Egypt with the feast of Passover. Each family is to take an unblemished lamb, slaughter it, take some of its blood and smear it on their doorposts, then they are to eat its flesh. We read of this in Exodus 12:8-11:

[Exodus 12:8-11 NKJV] Then they shall eat the flesh on that night; roasted in fire, with unleavened bread and with bitter herbs they shall eat it. Do not eat it raw, nor boiled at all with water, but roasted in fire—its head with its legs and its entrails. You shall let none of it remain until morning, and what remains of it until morning you shall burn with fire. And thus you shall eat it: with a belt on your waist, your sandals on your feet, and your staff in your hand. So you shall eat it in haste. It is the LORD's Passover.

So unleavened bread is associated with the Passover celebration to signify the haste with which the children of Israel were to leave Egypt after the tenth plague. Then immediately after the celebration of Passover is the Feast of Unleavened Bread, a seven-day feast following on the heels of the Passover. During this seven-day feast, not only are the Israelites only to eat unleavened bread, they’re not to have any leaven at all in their homes. Consider Exodus 12:17-20:

[Exodus 12:17-18, 20 NKJV] So you shall observe the Feast of Unleavened Bread, for on this same day I will have brought your armies out of the land of Egypt. Therefore you shall observe this day throughout your generations as an everlasting ordinance. In the first month, on the fourteenth day of the month at evening, you shall eat unleavened bread, until the twenty-first day of the month at evening. You shall eat nothing leavened; in all your dwellings you shall eat unleavened bread.

When we come to the Book of Leviticus we see God give to Moses all of the rules and regulations that were to guide Israel’s religious observance. The early chapters of Leviticus give instructions for the various offerings that were to be performed, and one of them is the grain offering:

[Leviticus 2:4-5 NKJV] And if you bring as an offering a grain offering baked in the oven, it shall be unleavened cakes of fine flour mixed with oil, or unleavened wafers anointed with oil. But if your offering is a grain offering baked in a pan, it shall be of fine flour, unleavened, mixed with oil.

So any kind of grain offering made to God had to be with “unleavened cakes.” So now why all this emphasis on unleavened bread? Is there any significance to bread without leaven? The only clue from the OT we can draw is from Deuteronomy 16:3, which reads, “You shall eat no leavened bread with it; seven days you shall eat unleavened bread with it, that is, the bread of affliction (for you came out of the land of Egypt in haste), that you may remember the day in which you came out of the land of Egypt all the days of your life.” The unleavened bread is called the “bread of affliction” and is associated with Egypt. Outside of this, we’re not really given any hints in the OT as to the significance of unleavened bread. But we can infer from Deuteronomy 16:3 that anything associated with Egypt is bad. Egypt was the place of bondage. It’s representative of the world as opposed to God; everything that attempts to lure God’s people away from Him. Recall all the times during the exodus when the people complained. They angered God when they remembered the “good ol’ days” of Egypt.

We get a clearer picture when we get to the NT. Jesus uses the illustration of leaven to describe two things. First is the kingdom of heaven in Matthew 13:33. The illustration is to show how the kingdom of heaven will permeate society in this age much like leaven permeates a lump of dough. The second is to describe the teachings of the Sadducees and Pharisees in Matthew 16:6-12. Here leaven is seen negatively as the legalistic teaching of the Pharisees which can poison the spiritual life of God’s people. This theme is picked up by Paul in 1 Corinthians 5:6-8 to describe sin:

[1 Corinthians 5:6-8 NKJV] Your glorying is not good. Do you not know that a little leaven leavens the whole lump? Therefore purge out the old leaven, that you may be a new lump, since you truly are unleavened. For indeed Christ, our Passover, was sacrificed for us. 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.

What makes this passage helpful is that Paul links his teaching with the OT Passover celebration. The context of this passage is the situation in Corinth in which a man was having in illicit affair with his father’s wife. That was bad enough, but the Corinthians were celebrating it. Paul rebukes them and tells them they need to exercise church discipline on the man and thereby “purge out the old leaven.” So I think we can safely say that leaven is a picture of sin. Just as the Israelites were to celebrate their salvation from Egypt by removing leaven from their homes, we are to celebrate our salvation from sin and death through Christ by removing the leaven of sin from our lives.