When you read thru this verse, Jesus is warning his disciples then and now us to be on our guard. We all know that the Pharisee yeast is small bits of hypocrisy here, little distortion there, that can grow and grow you away from truth. Just like how a small amount of yeast causes a loaf of bread to grow and rise out greatly, unless it indeed is left too long and the bread collapses (hmm...maybe any uncovered hypocrisy that stacks up in our lives will lead up to the collapsing of our lives or faith..) I went to Wikipedia and read up on yeast and found something cool. Yeast does not need light to grow...neither does hypocrisy. Lets be a people in the light.

Luke 12:1-3