Be on your guard against the yeast of the Pharisees and the yeast of Herod

 Mark 8:14-21 ©

The disciples had forgotten to take any food and they had only one loaf with them in the boat. Then he gave them this warning, ‘Keep your eyes open; be on your guard against the yeast of the Pharisees and the yeast of Herod.’ And they said to one another, ‘It is because we have no bread.’ And Jesus knew it, and he said to them, ‘Why are you talking about having no bread? Do you not yet understand? Have you no perception? Are your minds closed? Have you eyes that do not see, ears that do not hear? Or do you not remember? When I broke the five loaves among the five thousand, how many baskets full of scraps did you collect?’ They answered, ‘Twelve.’ ‘And when I broke the seven loaves for the four thousand, how many baskets full of scraps did you collect?’ And they answered, ‘Seven.’ Then he said to them, ‘Are you still without perception?’

The "yeast" or "leaven" of the Pharisees is a subtle but pervasive influence they have over people. It can be understood as self-righteousness, spiritual pride, and hypocrisy. 

In the Bible, yeast is usually understood to symbolize something negative. Jesus uses the analogy of yeast to teach a spiritual lesson about how a small amount of yeast can leaven dough thoroughly. The analogy is that those who follow the Pharisees might demand signs, but they would gradually increase in unbelief until they had hardened hearts.