Mark 8:14-21 NLT
 But the disciples had forgotten to bring any food. They had only one loaf of bread with them in the boat.  As they were crossing the lake, Jesus warned them, “Watch out! Beware of the yeast of the Pharisees and of Herod.”  At this they began to argue with each other because they hadn’t brought any bread.  Jesus knew what they were saying, so he said, “Why are you arguing about having no bread? Don’t you know or understand even yet? Are your hearts too hard to take it in?  ‘You have eyes—can’t you see? You have ears—can’t you hear?’ Don’t you remember anything at all?  When I fed the 5,000 with five loaves of bread, how many baskets of leftovers did you pick up afterward?” “Twelve,” they said.  “And when I fed the 4,000 with seven loaves, how many large baskets of leftovers did you pick up?” “Seven,” they said.  “Don’t you understand yet?” he asked them.
Reflection . . .
Jesus tells his disciples to “Beware of the yeast of the Pharisees and of Herod.” Why does he say that in this interaction with his disciples?
Just a few verses before this event, at the beginning of the chapter we have the story of the Feeding of the 4,000. They are still thinking about that miracle and realizing that they only had one loaf of bread with them in the boat while crossing the lake. Jesus warns them.
What is it that Jesus’ disciples didn’t understand?
They didn’t understand that life is so much more that physical things, it’s about spiritual things that matter. They need not worry about not having enough bread to eat, what they needed to be concerned about was the the damage the Pharisees and Herod were doing to the spiritual environment in which they were ministering. They were the greatest opposition to their ministry. They were the very thing that was preventing the people from seeing the truth, the very spiritual nourishment that was needed.
The “yeast” of their opponents was their arrogant attitudes and resistance to the truth. That was what Jesus’ disciples didn’t grasp.
Are we concerned about what keeps people from understanding and receiving the Gospel? We should be. Prayer and obedience to the Lord is how we stand against these forces and help people.