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, yeast 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 yeast that oozed out beyond the boundaries of its container.

In the Gospel, Jesus warns his disciples to guard against the yeast of the Pharisees and the yeast of Herod. In Luke, the yeast of the Pharisees is hypocrisy (Lk 12:1). Here, the yeast of the Pharisees points to their evil disposition, to their hardness of heart: 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 yeast is in their hostility and unbelief. Herod, on the other hand, seeks Jesus to see a sign/ miracle from him (Lk 23:8). He wants to be entertained and awed. The yeast 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.

Paul refers to the condition of persons before they turn to Christ as the “old yeast” that becomes pervasive of the whole person. It is the yeast of malice and wickedness. Now that they have become Christians, they become “unleavened bread” (used for the celebration of the Passover and the Eucharist). Now they must clear out the old yeast and replace it with a fresh batch of dough, the unleavened bread of sincerity and truth (1 Cor 5:7-8).