Another parable spake he unto them; The Kingdom of heaven is like unto leaven, which a woman took, and hid in three measures of meal, till it was all leavened. Matthew 13:33
Only bakers really appreciate the usefulness and function of yeast. Bread made without it stays flat and compact, and it is not particularly enjoyable to eat unless baked in thin sheets. Yeast in dough reacts with the starches and produces carbon dioxide, which causes the whole ball of dough to expand and rise. When baked, the yeast dies, but the air pockets remain, creating fluffier and tastier bread.
That’s how the Spirit works among people. Jesus “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.” (Matthew 13:33) When yeast is kneaded into dough, it is invisible; nothing happens right away. But in its own time the dough expands and rises, driven by the yeast.
In the same way we are not able to see the Spirit as he moves through groups of people. Sometimes it may seem as if nothing is happening, but when you look at the steady growth of the church from the handful at Jesus’ time to the hundreds of millions of believers today, all you can do is marvel.
God doesn’t want us thinking that we can do his job, and he absolutely refuses to do ours. We can’t convert people but can certainly share the yeasty Word that does.
From ‘Serving God and Others: Devotions From Time of Grace