Adam Clarke on Matthew 13:33

The kingdom of heaven is like unto leaven - On the nature and effects of leaven, see the note on Exo 12:8. As the property of leaven is to change, or assimulate to its own nature, the meal or dough with which it is mixed, so the property of the grace of Christ is to change the whole soul into its own likeness; and God intends that this principle should continue in the soul till all is leavened - till the whole bear the image of the heavenly, as it before bore the image of the earthly. Both these parables are prophetic, and were intended to show, principally, how, from very small beginnings, the Gospel of Christ should pervade all the nations of the world, and fill them with righteousness and true holiness.