Now when His disciples had come to the other side, they had forgotten to take bread. 6 Then Jesus said to them, “Take heed and beware of the leaven of the Pharisees and the Sadducees.”
7 And they reasoned among themselves, saying, “It is because we have taken no bread.”
8 But Jesus, being aware of it, said to them, “O you of little faith, why do you reason among yourselves because you have brought no bread? 9 Do you not yet understand, or remember the five loaves of the five thousand and how many baskets you took up? 10 Nor the seven loaves of the four thousand and how many large baskets you took up? 11 How is it you do not understand that I did not speak to you concerning bread?—but to beware of the leaven of the Pharisees and Sadducees.” 12 Then they understood that He did not tell them to beware of the leaven of bread, but of the doctrine of the Pharisees and Sadducees. — Matthew 16:5-12.
Jesus frequently spoke in parables, but was misunderstood here by His disciples who thought He was speaking literally (v. 7). They should have realized from His feeding miracles that Jesus could provide food if that was what was needed, and so recognized that He was speaking metaphorically about the “leaven” (the doctrine/ teaching, v. 12) of the Pharisees and Sadducees.
False teaching, like leaven in dough, can quickly permeate the church and corrupt its holiness.