When I was growing up, our family summer vacations usually involved camping. One of those vacations was a five-week trip from Pennsylvania to California and back, with stops in the Badlands, Colorado, Oregon, Disneyland, and all sorts of places I don't remember all of these years later (but of course I do remember ballgames in Anaheim and St. Louis). We were pretty frugal on these trips--not a lot of restaurant meals, but usually dinner cooked in the campsite on the Coleman stove, breakfast before we packed up and moved on, and lunch being sandwiches and Kool-Aid at a picnic table at a roadside stop on the way somewhere.
Once we stopped for lunch and dived into the sandwiches and the Kool-Aid, and the Kool-Aid was undrinkable. It turned out that, since sugar and salt look nearly identical, one can be substituted for the other without noticing. Although salt mixes with Kool-Aid mix and water something like sugar, it is a poor substitute for sugar. That Kool-Aid had to be poured out--it tasted horrible, and certainly didn't quench a thirst!
Having the right mixture of ingredients matters. And in this parable that Jesus tells, it seems clear that certain ingredients can make a big difference in the results, since without yeast, the bread doesn't rise.
It is possible to make bread without yeast--that is what the Israelites fleeing Egypt had to do, since they didn't have time to wait for the process of the bread rising. To this day, "unleavened bread" is used each time Jews commemorate the Passover. Most of those listening to Jesus in real time would know that story, and the celebration; they would also know what the ingredient was that was missing in that unleavened bread. They would know the difference when the yeast is in the mix, to leaven the bread. Many, if not most, of them made bread at home. They would completely understand how leaven worked, what yeast did.
With their common knowledge of yeast and the difference it makes in making bread, to compare the Kingdom of Heaven/Kingdom of God to that yeast got them to think about how a seeming small thing that makes a big difference is at work in the world. It is taken for granted when the yeast is mixed in what will happen--and yet what happens is very different from when there is no yeast to make it happen. Unleavened bread looks different, feels different, and even tastes different, than leavened bread, and yet most of the time we take for granted the reality of leavened bread. Do we take for granted the way that the Kingdom of God is already at work in the world? Do we take for granted the role that we who follow Jesus not only can have, but may already have?
GOSPEL Matthew 13:33
33 He told them another parable: ‘The kingdom of heaven is like yeast that a woman took and mixed in with three measures of flour until all of it was leavened.’