At the time of Jesus' disciples crossed the lake, they forgot to take bread.
Jesus said to them, Be careful and on your guard against the leaven (ferment) of the Pharisees and Sadducees.
Hearing the words of Jesus, his followers began to say to one another, "He was just saying that because we did not bring bread."
But Jesus, aware of this, asked, Why are you discussing among yourselves the fact that you have no bread? O you [men, how little trust you have in Me, how] little faith!
Do you not yet discern (perceive and understand)? Do you not remember the five loaves of the five thousand, and how many [ small hand] baskets you gathered?
And what about the seven loaves for four thousand people? How many baskets of surplus food that you collect?
How is it that you fail to understand that I was not talking to you about bread? But beware of the leaven (ferment) of the Pharisees and Sadducees.
Then they discerned that He did not tell them to beware of the leaven of bread, but of the teaching of the Pharisees and Sadducees.