In the stories about Jesus, the disciples are often portrayed as the “everyman”—the relatable characters—the guys who always need things explained to them. It’s quite common that the disciples make asses of themselves by just not getting it, despite all the things they have already seen.
Straight after Jesus has fed 4,000 people with seven loaves of bread and some fish (not to be confused with the time He fed 5,000 people with five loaves and some fish), He and His disciples get in a boat to escape the crowds. It’s at this point that Pharisees and Sadducees turn up and start to hassle Jesus. Jesus quickly shuts them down and sails away.
On the boat, the disciples realise they forgot to bring any bread to eat.
Right at that moment, Jesus, somewhat metaphorically, warns them “Watch out and beware of the yeast of the Pharisees and Sadducees.”
This deeply concerns the disciples, and they start discussing the ramifications of their bread-forgetfulness. Jesus overhears.
“Don’t you understand yet?” He asks, bewildered. “Don’t you remember the five loaves for the 5,000 and how many baskets you collected? Or the seven loaves for the 4,000 and how many large baskets you collected?”
Jesus continues, “Why is it you don’t understand that when I told you, ‘Beware of the yeast of the Pharisees and Sadducees,’ it wasn’t about bread?”
The author has the final word: “Then they understood that He did not tell them to beware of the yeast in bread, but of the teaching of the Pharisees and Sadducees.”