Matthew 16:11

“How is it that you do not perceive that I did not speak to you about bread? But beware of the leaven of the Pharisees and Sadducees.”

So he asked them how they could possibly have failed to recognise that He was not literally taking about bread. Rather He was saying, ‘beware of the leaven of the Pharisees and Sadducees, that is of their harmful counter teaching. The Pharisees and Sadducees differed on many things, but certain things they were agreed on. They did not want to disturb the status quo, they did not believe that He was the Messiah, and they were agreed that if the Messiah came He would perform miraculous and spectacular signs. It is not simply accidental that this will be followed by Peter's direct confession of Jesus as the Messiah. He has taken to heart Jesus' words.