“Leaven” is the yeast that is added to bread dough to make it rise. It only takes a little to affect the whole. Although the disciples at first took the Lord literally and thought he spoke of bread, they finally realized he was warning against the teaching of the “Pharisees and Sadducees.” The teaching of the Pharisees was to be avoided because, although they believed the whole Hebrew Bible, they added to the law layer upon layer of tradition, until no one could keep it. Their “leaven” would lead to legalism. The Sadducees, on the other hand, denied much of the Hebrew Bible, affirming only the books of Moses. They were more interested in political power than in God’s power. Their “leaven” would lead to liberalism. Jesus warned his disciples to avoid both extremes.