Jesus is warning his disciples to beware of the leaven of the Pharisees, Sadducees and the Herodians. They did not understand at first and thought he was talking about bread, but they finally understood that He was talking about the teachings (Doctrine) of the Pharisees, Sadducess and Herodians. Leaven or Yeast is often depicted as sin in the bible and no doubt becoming involved with the doctrines of these people would have been sinful.
