After leaving Gentile territory, which He rarely visited, Jesus is once again confronted by the leaders of His own people (Mat 16:1-12, cf. 15:1-20). This time they came testing Him by asking Jesus to show them a sign from heaven. As He did earlier, Jesus rebuked them, and called them evil for requesting a sign (cf. 12:38-42). He also indicated once again that only the sign of Jonah the prophet would be given them. Jesus had given Israel many signs, signs which both confirmed Him as their King and that the kingdom of heaven had come upon them. But they were not able to discern the meaning of those signs (16:3). After the confrontation, Jesus left the Pharisees and Sadducees.
Jesus then warned His disciples about the leaven of the Pharisees and Sadducees. The disciples thought He was speaking of bread since they had forgotten to bring some along. However, Jesus was speaking of the doctrine or teaching of the religious leaders of Israel. As a little leaven or yeast permeates a whole lump of dough, so a little false doctrine can permeate the whole person or a whole community of faith and corrupt it. Paul used the analogy when he confronted the church of Corinth for not removing from their membership an unrepentant adulterer.
Your boasting is not good. Do you not know that a little leaven leavens the whole lump? 7 Cleanse out the old leaven that you may be a new lump, as you really are unleavened. For Christ, our Passover lamb, has been sacrificed (1 Corinthians 5:6-7).