He told them another parable. “The kingdom of heaven is like leaven that a woman took and hid in three measures of flour, till it was all leavened.” (Matt 13:33, ESV)
In our last parable, Jesus described the massive growth of the kingdom of God in agricultural terms—that is, primarily directed toward the men of first-century Judea. Now, He makes basically the same point but to the women of first-century Judea, who often baked bread.
Leaven is any substance which, when added to dough, causes the dough to rise. The most common form of course is yeast, which is comprised of numerous small bacteria that spread throughout the dough and change it on a chemical level.
Here, Jesus tells the story of a woman taking leaven and hiding it within “three measures of flour”—that is, about 8-9 gallons of flour. All it takes is a tiny bit of yeast dumped into the huge container of flour, and the natural processes go to work: it is impossible to stop, and before long the entire 8 gallons of flour is leavened. It cannot be undone, and the changes are real and deep, down to the chemical level.
This follows upon the mustard seed analogy, by pointing out that once Christianity started, there is no known process to stop it. It quickly, invisibly, inexplicably, and unstoppably spreads until everything is affected. Again, to His small band of listeners, this had to be comforting: here He was telling the women as well as the men that His kingdom might be starting humbly, but that it could not be stopped from growing to overtake the entire world.
(Aside: one of the major reasons that some—including me—speculate Judas betrayed Jesus was disappointment. Judas may have been part of the Zealot sect of Judaism, which believed that the Messiah would physically overthrow the Roman Empire and set up a new and eternal Jewish kingdom on earth. You can almost imagine Judas sitting here, listening to Jesus talk, and misinterpreting this as a promise to overtake the world—and then later, when Jesus’ goal proved to be purely spiritual in nature, one can see how Judas may well have been angered and discouraged enough to take the bribe and betray the Good Shepherd.)
When I read this passage, though, I also get an application from a personal, believers’ standpoint, not just from the standpoint of the spread of the faith as a whole. Overwhelmingly, I get the sense in this passage of inevitability. When leaven is added, the change happens naturally, organically, with no further effort, and completely; the very chemical composition of the dough is changed. How similar this is to our experience as believers! How comforting to know that, once the Spirit gets involved, it is inevitable that we will be made into the image of God. (As Martin Luther said, sanctification is nothing more than becoming the person you already are, and just don’t yet know it.) The Spirit begins working in us like leaven, spreading, changing, rewriting our very core composition, until one day we are hopelessly and completely and utterly His.