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.”
This short little parable from Jesus is one of the quickest stories He told. And it can be difficult to understand. Parables as a rule make just one main point, and in the case of this one, it is made for us right at the end: all the flour is permeated with leaven (yeast). It seems Jesus is emphasizing the quiet, nearly indiscernible influence of the kingdom of God in the larger world.
This point is heightened by the unusual way the yeast is added to the flour. This is not a story about a recipe for bread making. The woman did not "add" the leaven to the dough like a step in a recipe. She "hid" it in about fifty pounds of flour (which is what three measures works out to from the text). I've never heard that aspect mentioned much, but it seems important to the short story. This isn't about baking bread or making dough. There is something strategically off-center going on with the hiding of the leaven in that much flour. Hiding yeast in flour is not a good idea unless you are going to be making LOT of bread and only bread. The flour can only really be used for bread once it is leavened.
In the story, the woman hid yeast in the flour until all of it was leavened. She "took" it and "hid" it... almost like an intentional conspiracy is at work here. The kingdom of God grows through intentionally leavening the flour. Like the metaphors of salt and light that Jesus used in the Sermon on the Mount, leaven in this short parable emphasizes how just a little kingdom influence can by used by Christ to do a tremendous amount of gospel good in the world.
I believe the best gospel work is done by intentional conspiracy. Grant me that term, OK? We aren't public phoney-baloney Pharisees blowing trumpets to announce our good works. Instead, christians are like that desperate woman who secretly hid yeast in the flour so that it can only become bread! We are doing a kind of gospel espionage to the world system by preaching Christ and serving Him, caring for the neediest and loving people with His love. And that is an appealing picture of what Christ is doing in His kingdom.