Mk 8:14-21
14[The disciples] had forgotten to bring bread, and they had only one loaf with them in the boat. 15[Jesus] enjoined them, “Watch out, guard against the leaven of the Pharisees and the leaven of Herod.” 16They concluded among themselves that it was because they had no bread. 17When he became aware of this he said to them, “Why do you conclude that it is because you have no bread? Do you not yet understand or comprehend? Are your hearts hardened? 18Do you have eyes and not see, ears and not hear? And do you not remember, 19when I broke the five loaves for the five thousand, how many wicker baskets full of fragments you picked up?” They answered him, “Twelve.” 20“When I broke the seven loaves for the four thousand, how many full baskets of fragments did you pick up?” They answered [him], “Seven.” 21He said to them, “Do you still not understand?”
LEAVEN OF THE PHARISEES AND OF HEROD: The metaphor of leaven/yeast (Greek zume) turns on the ability of a small amount of yeast to impregnate the material with which it is mixed with its own fermentation. In both Jewish and Hellenistic cultures, leaven was a common metaphor for corruption. The Jews thought as a rule that things that were outside their proper limits were impure, and this was most evident in leaven that oozed out beyond the boundaries of its container.
In the Gospel, Jesus warns his disciples to guard against the leaven of the Pharisees and the leaven of Herod. The leaven of the Pharisees points to their evil disposition: they ask for a sign from Jesus when they have already judged Jesus to be not an agent of God but an agent of the demon. Their leaven is in their hostility and unbelief. Herod, on the other hand, seeks Jesus to see a sign/miracle from him (cf Lk 23:8). He wants to be entertained and awed. The leaven in his case is the disposition to believe only if signs which compel faith are produced. In contrast, Jesus calls for faith and understanding apart from signs.