Then they understood that He was not telling them to beware of the leaven used in bread, but of the teaching of the Pharisees and Sadducees.
The disciples finally understand that he meant the teaching of the Pharisees and Sadducees, not bread.
This verse resolves the misunderstanding by naming the teaching as the real warning.
5When they crossed to the other side, the disciples forgot to take bread. 6“Watch out!” Jesus told them. “Beware of the leaven of the Pharisees and Sadducees.” 7They discussed this among themselves and concluded, “It is because we did not bring any bread.” 8Aware of their conversation, Jesus said, “You of little faith, why are you debating among yourselves about having no bread? 9Do you still not understand? Do you not remember the five loaves for the five thousand, and how many basketfuls you gathered? 10Or the seven loaves for the four thousand, and how many basketfuls you gathered? 11How do you not understand that I was not telling you about bread? But beware of the leaven of the Pharisees and Sadducees.” 12Then they understood that He was not telling them to beware of the leaven used in bread, but of the teaching of the Pharisees and Sadducees.