What does Matthew 16:11 mean?

Jesus drives home His point about the small faith of the disciples. He has given a warning of enormous importance: to avoid "the leaven" of the Pharisees and the Sadducees. By their response, Jesus' followers have revealed they are preoccupied with something of tiny importance: they didn't bring bread for the trip. Their anxious focus on the lack of bread caused them to misunderstand Jesus' vital teaching (Matthew 16:5–10).

The fact that it's bread, of all things, which trips these men up is part of Christ's frustration. In recent verses, Jesus reminded them about the two occasions where He turned tiny amounts of bread into enough for thousands of people. These men were present, handed out bread, and they gathered up leftovers in both instances (Matthew 14:13–21; 15:32–39). Of all the people alive on the earth at that moment, these twelve men should have been most reassured that Jesus could provide bread for them whenever it was needed.

This gives us context for Jesus' clear frustration: how can you not understand this? Of all the spiritual metaphors to misinterpret, this (Matthew 16:6) is not one the disciples should have missed. Before we sneer at the disciples, however, we should keep in mind how often we all do the same. Focused on our anxiety, we sometimes miss what God was providing for us in the moment.

To be sure the lesson is not lost, Jesus repeats the symbol. Leaven, or yeast, was often used as a metaphor for evil in Jewish teaching. The reason for this is yeast's ability to totally saturate a large quantity of dough, despite being very small. The effect of that tiny bit of yeast is dramatic. Speaking of sin as "leaven" emphasizes this danger. Even the "tiny" wrong teachings from the Jewish religious leaders would grow, and had grown, to lead a generation of Israelites away from faith in the Messiah.