Here is a word of Jesus from Matthew 13: “33 He told them still another parable: ?The kingdom of heaven is like yeast that a woman took and mixed into a large amount of flour until it worked all through the dough.?
This parable is attached to the parable of the mustard seed, and the big issue has to do with whether or not these parables are parables of “growth” (describing how the kingdom will grow and eventually take over the world) or parables of “contrast” (describing humble beginnings as contrasted with great endings).

New book: Klyne Snodgrass has for twenty some years been working on a handbook on the parables of Jesus and it is now out (Stories with Intent). It’s a must-have for students and pastors. I’ll be posting on it later.
1. To compare kingdom of God to leaven is an oddity. Leaven (not yeast; leaven is fermented dough, a patch left to be used in the next batch) is not only unclean but it is also so insignificant, and this seems to be a major point of Jesus: it looks small, it looks humble, it looks insignificant.
2. To what would Jesus have been referring to, if we think concretely? The kinds of followers around him, the kind of background he had, the kind of ministry he carried out, the kind of response he was getting, and the kind of death he would die.
3. Leaven permeates, so the parable must also be referring to the permeative potency of the kingdom of God Jesus embodies and spreads.
4. There is here an organic growth as well — slow, painless, inevitable, keeps on working.
To what does the kingdom refer here? The power of affects and effects by Jesus and his kingdom message on those around him. Which means, there is also a growing number of followers around him — growth is about growth.