Among the Hebrews, leaven (or yeast) was very important. It had a practical use in making bread and a symbolic significance in religious rituals. People made leaven by mixing the flour of certain cereals with water, and allowing the mixture to stand till it fermented.
When making leavened bread, they mixed this fermented portion with dough, so that when the dough was baked in the oven the bread would rise (Matt 13:33; 1 Cor 5:6). Leavened bread was light and rounded, unleavened bread heavy and flat. An easier way of making leaven for future batches of bread was to remove a small piece of leavened dough before baking and leaving it stand till it too fermented. The first mention of any ritual significance of leaven was at the time of the Passover when Israel escaped from Egypt. During the week after the Passover escape, the people had no time to bake their bread leavened.
They had to carry their dough and baking pans with them, baking as they went (Exod 12:11,18,34,39). Each year from that time on, the people were to hold a symbolic re-enactment of the Passover along with a week-long Feast of Unleavened Bread. This was to remind them of Israel’s hurried and unceremonious departure from Egypt. They were to leave no leaven in their houses during the week of the feast (Exod 12:14-20; 23:15; Mark 14:1; see PASSOVER). Nothing containing leaven was to be offered on the altar of sacrifice. This was probably because leaven spoiled easily, and there was to be no trace of corruption in the sacrifices (Exod 23:18; 34:25; Lev 2:11; 7:12; 10:12). However, leavened bread, representing the ordinary food of the people, was presented to God at the Feast of Harvest, as an expression of gratitude to him for their daily food (Lev 23:15-20).
Leavened bread offered with the peace offering was not burnt on the altar, but eaten in the meal that followed (Lev 7:11-14). Because it tended to corrupt and because it affected everything it touched, leaven developed a deeper symbolic meaning. Jesus saw the Pharisees, the Sadducees and Herod as evil influences that spread through Israel as leaven spreads through a lump of dough. He warned his disciples to beware of the leaven-like effect of such people. Their hypocrisy, teaching and ungodliness could quickly have a corrupting effect on others (Matt 16:5-12; Mark 8:15; Luke 12:1-3). Just as Israelites cleaned all leaven out of their houses at the time of the Passover, so Christians should clean the leaven of sin and wrong teaching out of their church. If left unchecked, sin will spread yeast-like through the church (1 Cor 5:6-8; Gal 5:7-9). In one of Jesus’ parables, by contrast, leaven is used figuratively in a good sense. Just as leaven spreads through the dough into which it is put, so will Christ’s kingdom spread throughout the world (Matt 13:33).