In scientific terms, yeast is a fungus that ferments carbohydrates and makes dough rise. It is a key ingredient in the making of bread, beer, wine, and spirits. Leavened bread is present in most cultures from prehistoric times onward, including that of the Israelites. That is why Moses’ instruction to leave yeast out of their bread in preparation for the exodus seemed notable and became part of the remembrance celebration of the Passover. BREAD IN THE OLD TESTAMENT
During Passover, leavened bread could not be eaten or stored in the houses of the Israelites for seven days (Exod 12:15-20). This act was a symbolic remembrance of the first Passover, when God saved the firstborn of the Israelites who had put blood over their doors, but killed the firstborn of the Egyptians so that Pharaoh would let the people go. Omitting the leaven made the bread faster to prepare and easier to transport as the Israelites hurried out of Egypt. What began as a practical consideration became a symbol of the event.
Later on, when the Israelites were established with the laws given by God, bread took on new meaning. leavened bread was presented as a peace offering (Lev 7:13). It was also included in the sacrifice of the firstfruits (Lev 23:17). In these uses, bread signified the provision of God. It was the most basic of foods, the one for which Jews gave thanks as they acknowledged that their needs were met by Jehovah. Jesus continued this image of bread as provision when he told us, in the Lord’s Prayer, to be thankful for our daily bread (Matt 6:11). These daily uses of leavened bread made the symbolism of unleavened bread all the more poignant as the wafers at Passover were reserved for a celebration of remembrance and reflection.
YEAST AS INFLUENCE
The physical properties of yeast that make it multiply and cause dough to rise provide an image of the influence that permeates population, either positively or negatively. Jesus’ parable about the growth of the kingdom of heaven illustrates how the good news of the gospel will permeate the world: “The kingdom of heaven is like yeast that a woman mixed into a large amount of flour until the yeast worked its way through all the dough” (Matt 13:33; see also Luke 13:20-21). Individual believers will affect their sphere of influence, multiplying believers. Then, by working together, they will transform entire nations.
On the other hand, Jesus also instructs us to beware of the leaven of the Pharisees, Sadducees, and Herod (Matt 16:6, 11; Mark 8:15). Their negative influence can spread like yeast throughout a population and gradually poison it. Paul offers a similar warning, saying that ignoring sin in the congregation can affect a church body in the same way yeast makes its way through all the dough.
Don’t you know that a little yeast spreads through the whole batch of dough? Remove the old yeast of sin so that you may be a new batch of dough, since you don’t actually have the yeast of sin. Christ, our Passover lambs, has been sacrificed. So we must not celebrate our festival with the old yeast of sin or with the yeast of vice and wickedness. Instead, we must celebrate it with the bread of purity and truth that has no yeast. (1 Cor 5:6-8; see also Gal 5:9)
The Corinthians needed a new batch of bread dough to rid themselves of the false teaching that had spread through the congregation.
As we look back at the Old Testament’s forbidding of yeast during the Passover and then look forward to the New Testament’s use of yeast as a symbol for a person’s influence for good or evil, we see a similar emphasis on the need for purity in the body of Christ. The Passover was a tangible, historical event in which God’s people had to be set apart and made ready for their journey to an earthly kingdom. Unleavened bread was a crucial part of their preparation. In New Testament, the preparations Christians make are spiritual and they are for a future spiritual kingdom. But keeping the leavening pure in the body of Christ is just as important as the physical elimination of yeast during the Passover. The encouragement to spread the kingdom in people’s hearts and the warnings to avoid false teachings and sin that invade the body of Christ hearken back to the preparation of God’s people to enter the Promised Land.