He told them another parable: "The kingdom of heaven is like yeast that a woman took and mixed in with three measures of flour until all of it was leavened."

Traditionally yeast had been viewed as an impurity.  It could not be present in temple sacrifices.  "No grain offering that you bring to the Lord shall be made with leaven." (Leviticus 2.11)  Jesus tells his disciples "beware of the yeast of the Pharisees," (Mark 8.15) their malicious influence always seeking to spread out into society at large and must be guarded against.

The feast of Unleavened Bread was celebrated since the time of the Passover (and indeed had even earlier roots in Canaan as an offering of the first fruits of the harvest) as "commemorating the deliverance of the people of God the 'first-fruits,' as it were, of the nations."  (The Navarre Bible Commentary, Pentateuch, p 289)

Can we not meditate on leaven then as the flight from the slavery of sin in Egypt into the promised land which already has its fulfillment in Christ?  Jesus declares that the kingdom has in a way already come (Luke 11.20, Luke 17.21) and so leaven takes on a whole new positive meaning in Christ, his Spirit moving out through the whole earth offering, already now, a foreshadowing of the fulfillment and abundance which will be ours, in hope, in heaven.  Christ is leaven, and we are meant to be leaven too.  Leaven, salt, light.  But leaven's influence is silent, wondrous, mysterious.  Present yet invisible, strongly influencing all the flour.