What does Matthew 13:33 mean?

This parable is not so much a story as a snapshot of an ordinary event from everyday life. Most people would have seen a woman making bread. That process included adding leaven, usually yeast, to flour. Leaven is a fermenting agent that causes dough to rise over time. Jesus' point with this parable is closely related to the previous parable of the mustard seed (Matthew 13:31–32). Something seemingly tiny and obscure spreads, and grows, until it is something relatively enormous.

In this metaphor, a small amount of leaven is mixed into or "hidden" inside three measures of flour. This would have been about 50 pounds, or 23 kilograms. This would create enough bread to feed about 100 people. Jesus compares the kingdom of heaven to the leaven, the agent that causes the flour to grow into abundant bread.

Again, Jesus' description of the kingdom of heaven as a small thing that grows over time would not have fit the expectations most Israelites' held for the promised kingdom. Jesus is showing that the kingdom will begin in the hearts of His followers and only eventually grow to become the glorious kingdom the people were hoping for.

Alternatively, some read this parable differently, understanding the leaven or yeast to represent evil. Leaven is often used in that way in stories or analogies throughout the Old and New Testaments, though not always. If that was Jesus' intent, then the leaven in this parable would be like the weeds in the parable about the wheat. The leaven would represent the existence of evil alongside those who would come to trust in Jesus as King and Savior. Those evil ones, however, are doomed to be eradicated by Jesus at the judgment before He establishes His political kingdom on earth (Matthew 24:41–43). Preventing this interpretation, however, is the fact that Christ does not refer to leaven infiltrating the kingdom. Rather, in this parable, leaven is the kingdom.