2/1 Mark 8:15
15 And he cautioned them, saying, "Watch out; beware of the leaven of the Pharisees and the leaven of Herod."
The Jewish people referred to leaven as a picture of sin. It was an illustration of pride. Sins such as gossip, slander, anger, or false teaching spread from person to person, similar to the way a once small quantity of dough spreads into a large lump that can overflow the bowl as the leavening takes effect. Those sins often cause us to put ourselves above others in our own estimation. That is pride. Jesus told His disciples to watch out for the leaven of the Pharisees and the leaven of Herod.
The disciples were thinking Jesus was somehow referring to the fact that they had only brought one loaf of bread with them. However, Jesus was speaking of the two different sins, each of which resulted in pride that was spiritually disabling. The leaven of the Pharisees referred to exalting themselves above others because they lived strict lives of discipline in an attempt to follow all the Laws of Moses and everything written about those laws. They didn't want common people to touch them because they considered themselves so holy. The leaven of Herod and those Jews who backed him, the Herodians, was that of compromise with the world. They wanted the power of being the rulers and the freedom to worship Greek gods when they were with the Greeks. Their pride was in thinking they were above others and could compromise the Law of Moses because they were in power.
These two factions were polar opposites, and yet the seduction of thinking oneself more important than others made them similar. Each group expected respect and honor from man. Jesus was warning the disciples of this dangerous temptation. Just because they were following the Messiah did not make them more valuable to God than any person they met. Jesus was warning them not to be prideful that God had chosen them to be apostles, but to relate to everyone as a fellow human created in the image of God.
Consider: We must watch out for the leaven of self-exaltation. If we have more to share spiritually or physically, it is only because of the grace of God. The homeless person on the street, the senator from our district, or the pastor in our church are each made in the image of God. Every fallen person is only saved by the grace of the Savior. God loves each of them and we should, too.