And they began reasoning among themselves, saying, "[Why be concerned about yeast since] we did not bring [any] bread?"

How is it that you do not understand that I was not speaking to you about [physical] bread? But [rather I meant] watch out for the yeast [i.e., the influence] of the Pharisees and Sadducees."

Then they understood that He was not telling them to watch out for the yeast in [physical] bread, but for the teaching of the Pharisees and Sadducees.