This short parable of the leaven is talking about the kingdom of God within you. It is similar to the previous parable of mustard seed, but there are some marked differences. There are many commentators who have placed a negative connotation on this parable of the leaven because Jesus uses the analogy of leaven as something evil in other places. How ever in this scripture he is comparing this analogy to the kingdom of heaven, which is good, and so it should be seen in the light of something good.
Leaven is important to the making of bread because it causes the bread to rise and makes it soft and palatable. The way leaven works is through an organic process. As it sits in the flour mixture it slowly expands and works its way through the flour until all of the flour becomes activated by the yeast or leaven. Leaven improves the quality of the bread.
In the parable of the leaven we see a more personal view as it shows the kingdom of god is within you and how the gospel works to transform you. First we see a woman “hides” a piece of leaven, which is yeast or yeast affected dough, in three measures of flour. The act of hiding this leaven is interesting and the process of hiding something is seen in other places as well. We see in verse 44 of this chapter that a man found a hidden treasure and covered it up till he could acquire the field and own it fully. Similarly this woman hid the leaven in the flour.
When a person hears the words of the kingdom, if they do not fully understand it, the evil could snatch it away if they are not careful. This was like the birds taking the seed in the parable of the sower. (Matthew 13:4, 19) Thus by hiding it they can meditate on it and learn more so that the word expands and grows in them, transforming them in the same way that leaven transforms flour to make bread. It is a slow growth as the words of the kingdom of God work on your life.
Like the leaven is in the flour, the words and the truth of the kingdom of God is within you. The teachings of Jesus act on your life from within, working from the inside out. The work of transformation is a work that occurs by the Holy Spirit on your own spirit. It starts within and changes your life from within, just as leaven transforms flour from within. This work of transformation is a long slow process, which is also why we speak of walking with Jesus. We do not run with him but we walk. Walking is sustainable for a long time, and suggests a slow but steady progression to reach a destination. Leavening a lump of dough is also a slow process and progression until it is fully ready for baking. And we are being slowly transformed by the working of the truth of the gospel as we grow from within until we are changed into the image of Jesus.