After comparing the Kingdom to a Mustard Seed, Jesus teaches a parable about leaven hidden in a lump of dough.
Like a mustard seed, a lump of leaven (ζύμη) is small and hidden into fresh dough. Like a sourdough starter, leaven is naturally present in flour. When you knead the leaven into flour and water, the leaven will ferment. If you let the dough sit for seven days, the dough will begin to rise. Then that starter batch can be worked into more dough to bake bread. There is a difference between leaven and yeast; yeast in the modern sense is a leavening agent that you can buy in a store as a dry powder. Leaven in the ancient world is simply fermenting dough.
Like the mustard seed growing into a tree, there is hyperbole in the Parable of the Hidden Leaven. This woman is making a very large quantity of bread. “Three measures is the equivalent of almost 40 liters, enough for a meal for more than 150 persons or for approximately 110 pounds of bread” (Luz, Matthew 8-20, 262). About four pounds of leaven would be needed! This is not normal story about a woman baking bread for her family, it is a shocking hyperbole.
Leaven and Sin?
One problem with the parable of the Hidden Leaven is that in the rest of the Bible, leaven or yeast is a metaphor for sin. A week prior to the Passover, the Jews were to clean their homes and get rid of all the old leaven (Exodus 12:15, 19). This is called the Feast of the Unleavened Bread and explains the tradition of matzah bread at the Passover meal. Any bread made for Passover would not have any leaven in it and would therefore not rise. Cleaning out all the old leaven from the house represents removing sin before the Passover.
In Matthew 16:6-12 Jesus will warn his disciples about the leaven of the Pharisees and Sadducees, referring to their teaching (16:12). In the overall context of Matthew the leaven of the Pharisees is their hypocrisy.
Paul uses leaven as a metaphor for sin in the Corinthian church (1 Cor 5:6; cf. Gal 5:9). Like a little leaven in a lump of dough causes the bread to rise, even a little bit of sin corrupts the whole church.
The Hiddenness of the Kingdom of God
Rather than focusing on the leaven as an image for sin, Jesus’s emphasis is on the hiddenness of the leaven. What the kingdom is not at all obvious during the ministry of Jesus. Like a bit of leaven hidden in a lump of dough, Jesus’s announcement that the kingdom is present in his ministry is not at all what the Jewish people were expected from the Messiah. When the kingdom comes, it comes all at once! It is the hole loaf, or the fully grown tree from the very start.
Like the tree in the previous parable, when the kingdom fully arrives it will bless the whole world (like a woman who bakes 110 pounds of bread!) Like the birds building their nests in a tree, there is no need to allegorize the parts of the saying. The woman, the bread, the oven are all props in the story necessary to describe hidden leaven.
Parables and Millennialism
Do the parables of the mustard seed and the hidden leaven teach that the church is “building the kingdom of God”? (Or, to put this in systematic theology terms, is Jesus teaching post-millennialism?) Not at all. But these are the best verses in favor of post-millennialism. For example, Swete thought the leaven refers to “the kingdom’s subtle power of spreading itself through society and transforming it” (The Parables of the Kingdom, 43-44).
However, there is an already state of the kingdom (the seed, the leaven) and the not yet state of the kingdom (the tree, the loaf). There is no middle state of the kingdom, there is nothing here instructing the disciples to be the growing tree or the fermenting leaven.
As with the parable of the mustard seed, this parable encourages the disciples of Jesus who may have wondered (along with the Pharisees and others) how Jesus’s announcement of the kingdom of God could possibly be related to the glorious promises of the Old Testament.