In Matthew, Mark, and Luke, Jesus warned the disciples of leaven. Each passage is a little different, indicating that Jesus issued the warning more than once. Each passage warns of the leaven of the Pharisees. Matthew adds the Sadducees and Mark adds Herod. What were the disciples supposed to learn from the warnings? And what do they mean to us today, when the Pharisees, Sadducees, and Herod are all ancient history?