What does Matthew 16:7 mean?

Jesus and His disciples have travelled once more across the Sea of Galilee (Matthew 16:5). He seems to still be thinking about His most recent confrontation with the Pharisees and Sadducees (Matthew 16:1–4). The disciples are not. Instead, they have realized they forgot to pack bread and are concerned about that. Whether they spoke up about bread, Scripture does not say.

All we know is that Jesus told the disciples to "watch and beware of the leaven of the Pharisees and Sadducees" (Matthew 16:6). Leaven is an agent used to make bread rise, such as yeast. This was often used in Jewish teaching as a metaphor for evil. The substance itself is not unclean; the symbolism is because leaven is something seemingly small, which thoroughly infiltrates and changes whatever it touches. Christ is warning His disciples that the false teaching of the Pharisees and Sadducees could grow to overwhelm the truth as it was understood by the people.

For their part, the disciples' response could come across as comical, if Jesus did not react to it so harshly. They are pre-occupied with the most mundane of concerns: food. They have realized they forgot to pack bread for this trip. When they hear Jesus mention "leaven," they immediately conclude He is speaking about that. Or, that He's warning them not to eat something from those religious leaders.

Jesus' response will reveal that, even in this misunderstanding, the root issue is a lack of faith. He's provided bread before, so there's no reason for them to be so worried about food (Matthew 16:8–10).