In 8: 14 the word 'yeast' represents hidden evil influences. Jesus warned the disciples to be on their guard against the spiritually blind Pharisees and the unscrupulous power-seeking of those who supported the Herodian rulers of the Jews. Both the Pharisees and those in political power would join together in making plans to kill Jesus (3:6). Mark does not make it explicit what the disciples did not understand (8: 17, 21) but we can work it out. It has been clear throughout Mark's record that despite all the teaching the disciples have received from Jesus and all the miracles they have seen him perform, they still do not understand him. They also are spiritually blind because of their lack of faith.