Expositor's Bible Commentary Commentary Mark 8:15

"And he charged them, saying, Take heed, beware of the leaven of the Pharisees and the leaven of Herod." — Mark 8:15 (ASV)

Jesus warned his disciples about the yeast of the Pharisees. Yeast is a symbol of evil; just as only a very small amount of it can leaven an entire loaf of bread, so evil has a permeating power. Here the yeast of the Pharisees clearly refers to their desire for a sign from God to validate the actions of Jesus (similarly, the yeast of Herod). Jesus was appealing to his disciples to understand that the authority he possessed could not be proved by a sign. Only by faith could they recognize him as the bringer of God’s salvation.