Yeast of the Pharisees

The disciples are a little slow here again. They once again focus on the simplistic rather than understanding what Jesus is trying to say. They Have seen Jesus feed thousands two times now - but they still think that he is warning them about material things (lack of food). He once again has to explain that he is far more worried about their spiritual health than their physical health, and so explains that by yeast he is referring to something which in a small amount can spread throughout a large mass and totally change it. Jesus knows the effect that the Pharisees false teach will have on a believers walk.