He presented another parable to them: “The kingdom of heaven is like a mustard seed that a man took and sowed in his field. It’s the smallest of all the seeds, but when grown, it’s taller than the garden plants and becomes a tree, so that the birds of the sky come and nest in its branches.”
He told them another parable: “The kingdom of heaven is like leaven that a woman took and mixed into fifty pounds of flour until all of it was leavened.”
Of these two parables I've heard the mustard seed most discussed.
I think these parables are linked and are getting at a similar point. The mustard seed is small but it grows to be a very large plant. Relatedly, the leaven is a small contribution to the (fifty pounds! of) dough but it has very significant, expansive impact.
In teaching these parables, most people tend to dwell upon the magnificent effects of the mustard seed and the leaven. The Parable of the Mustard Seed is frequently connected to "faith the size of a mustard seed." But that's a different teaching from Matthew 17. Sure, the two references could be linked, but I think it's better to see the mustard seed reference in Matthew 13 as concerning the kingdom of heaven rather than about the size of our faith.
However, by focusing upon the impressiveness of the downstream effects we miss the point about the front-end smallness and insignificance and its impact upon the kingdom's reception. Again, I think a pessimistic note is being sounded here. Recall, Jesus is speaking in parables to sort his audience between those who see and those who don't see, between those who hear and those who don't hear. The parables effect a perceptual sorting. And why might the kingdom be misperceived? The Parables of the Mustard Seed and Leaven suggest that the kingdom, upon first encounter, appears small and insignificant. And that perception of inconsequentialness causes rejection and dismissal.
This is what I mean by "pessimism." The the kingdom of heaven, Jesus is saying, isn't all that impressive. Consequential, but not impressive. The kingdom, upon first encounter, is not going to knock anyone out or grab anyone's attention. The kingdom doesn't have a Wow factor. The kingdom is not trendy or buzzy. And because of this, the kingdom will be bypassed and overlooked.
Again, just like the Parables of the Sower and the Wheat and Tares, I'm struck here how the parables of Matthew 13 wouldn't fit into any church growth book or program. Not only is the kingdom of God generally unsuccessful (the Parable of the Sower) and morally ambiguous and compromised (the Parables of the Wheat and Tares and the Net), the kingdom's first impression isn't very impressive (Parables of the Mustard Seed and Leaven).
To remind, Jesus describes the kingdom as a "secret" and I can't help but wonder if a part of that "secret" is associated with this pessimistic thread running through the parables of Matthew 13. As I've shared, the reception of Jesus' parables are their very interpretation.
