Definition and Usage:
Yeast, also known as leaven, is a substance used to ferment dough, causing it to rise and become light and porous. In biblical times, leaven was typically a piece of dough retained from a previous batch, which had fermented and was used to leaven the new dough. The process of leavening is a natural fermentation that results in the production of carbon dioxide, which causes the dough to expand.

Symbolism in Scripture:
In the Bible, leaven is often used symbolically to represent influence, both positive and negative. It is frequently associated with sin and corruption due to its pervasive nature, as a small amount can affect a large batch of dough.

1. Negative Connotations:
· Sin and Corruption: Leaven is often used as a metaphor for sin and moral corruption. In 1 Corinthians 5:6-7 , Paul warns the Corinthian church, "Your boasting is not good. Do you not know that a little leaven leavens the whole batch of dough? Get rid of the old leaven, that you may be a new unleavened batch, as you really are. For Christ, our Passover lamb, has been sacrificed."
· False Teaching: Jesus warns His disciples about the corrupting influence of false teachings using the metaphor of leaven. In Matthew 16:6 , He says, "Watch out! Beware of the leaven of the Pharisees and Sadducees." Here, leaven represents the false doctrines and hypocrisy of the religious leaders.

2. Positive Connotations:
· Kingdom of Heaven: In a positive sense, leaven is used to illustrate the transformative power of the Kingdom of Heaven. In Matthew 13:33 , Jesus compares the Kingdom to leaven: "The kingdom of heaven is like leaven that a woman took and mixed into three measures of flour until all of it was leavened." This parable highlights the pervasive and transformative influence of God's kingdom in the world.

Leaven in Jewish Tradition:
Leaven holds significant importance in Jewish tradition, particularly in the observance of Passover. During this festival, the Israelites were commanded to remove all leaven from their homes and eat unleavened bread for seven days, commemorating their hasty departure from Egypt when there was no time for their bread to rise. This is detailed in Exodus 12:15 : "For seven days you must eat unleavened bread. On the first day, you are to remove the leaven from your houses. Whoever eats anything leavened from the first day through the seventh must be cut off from Israel."

Cultural and Historical Context:
In ancient Israel, leaven was a common household item, essential for daily bread-making. The process of leavening was well understood, and its symbolic use in Scripture would have been readily grasped by the original audience. The dual symbolism of leaven as both a corrupting and transformative agent reflects its dual role in daily life as both a necessary and potentially problematic substance.

Conclusion:
The concept of leaven in the Bible serves as a powerful metaphor for influence, whether it be the insidious spread of sin or the transformative power of God's kingdom. Its use in Scripture underscores the importance of vigilance against corruption and the potential for small beginnings to lead to significant change.