Now they had forgotten to take bread with them; they had no more than one loaf in the boat. Jesus began to warn them: "Beware," he said, "be on your guard against the leaven of the Pharisees and the leaven of Herod." They said among themselves, "Is is because we have no bread?" Knowing what was in their minds, he asked them, "Why do you talk about having no bread? Have you no inkling yet? Do you still not understand? Are your minds closed? You have eyes: can you not see? You have ears: can you not hear? Have you forgotten? When I broke the five loaves among five thousand, how many basketsful of scraps did you pick up?" "Twelve", they said. "And how many when I broke the seven loaves among four thousand? They answered, "Seven." He said, "Do you still not understand?" Mark 8: 11-21
Recall that before this gospel scene Jesus had fed 4000 people with seven little loaves of bread and a few fish. Then he had a brief encounter with some skeptical, scheming Pharisees before taking off with the disciples in a boat to the other side of the lake. Maybe as he embarked he said to himself, "Oh enough of this already."
But once they're in the boat the apostles start to fuss about having forgotten to take bread along with them. Sometimes our priorities are terribly messed up - even in religion. We shouldn't get defensive, this is after all a story about Jesus who asks some startling questions.
While the apostles spin emotionally about bread, Jesus starts talking to them about yeast. Yeast is symbolic of influence. Yeast influences flour quietly and invisibly while it makes the dough rise. Jesus knows what the anxiously distracted apostles are thinking and calls them out on it. Imagine! He warns them about the religious Pharisees (influencing yeast) who are filled with wrong ideas. Don't get infected by them. But the apostles don't understand because they're preoccupied with the lower place of their stomachs.
What's preoccupying me? How is my own mind closed? Has some wrong idea got hold of me? What bit of Christ have I forgotten or never paid attention to: "This justice stuff isn't for me," a proud parishioner once admitted.
These Pharisee guys are still very alive in hearts and minds.