Leaven (or yeast) symbolizes evil in 1 Corinthians 5:8 but growth in Matthew 13:33. It was used when baking bread to make bread rise. A little bit of leaven would be enough. Jesus is thus warning them against the bad influence and teaching of the Pharisees. The Pharisees were blind to spiritual things, seeing Jesus’ miracles but not responding in repentance and faith and then teaching others to stay away from Jesus. Herod was someone who loved the world and as a result he did not respond to John’s call to repentance (Mark 6:14–29). Jesus is warning his disciples not to follow in the ways of these two groups of people.
15 And he cautioned them, saying, “Watch out; beware of the leaven of the Pharisees and the leaven of Herod.”