What does Mark 8:15 mean?
Leaven is used as a metaphor for influence and growth (Galatians 5:9) either for evil (1 Corinthians 5:8) or good (Matthew 13:33). Both Jews and Greeks used the insidiousness of leaven as a symbol for corruption. Some believe that the leaven Jesus is referring to isn't the benign yeast we're familiar with but a leavening agent that has been infested with harmful bacteria.In Matthew's parallel passage, Jesus warns against "the leaven of the Pharisees and Sadducees" (Matthew 16:6). Luke 23:8 mentions that Herod, also, wants to see a sign, so it's reasonable to assume his followers, the Herodians, did as well. Earlier, the scribes join the Pharisees in demanding a sign (Matthew 12:38).
Pharisees and Sadducees were sects within the religion of Judaism. Pharisees held tightly to the Mosaic Law and the oral law, which they claimed God gave Moses to give more detail to His demands of the Israelites. Jesus argued with the Pharisees several times about their habit of putting manmade tradition above the needs of the people (Mark 2:15–28; 3:1–6; 7:1–23). The Sadducees rejected the oral law, which was good, but welcomed the influence of Greek and Roman culture, which was bad. They tended to seek positions of power in the secular realm, and usually controlled the Sanhedrin.
Scribes and Herodians were not religious sects. Scribes were scholars and lawyers who studied and interpreted the Mosaic Law. It was the scribes who added the manmade traditions that the Pharisees followed so carefully. Some, but not all, scribes were also Pharisees. If scribes were amplified Pharisees, Herodians were amplified Sadducees. Scholars believe they were a political party that supported Herod Antipas, ruler of Galilee and Perea, and the Roman Empire in general.
Only one thing can bring the Pharisees, Sadducees, scribes, and Herodians together and that is the threat Jesus poses to their equilibrium. The Pharisees and scribes resent how He so flagrantly dismisses their extra-biblical traditions. The Sadducees and Herodians fear He will bring political instability and they will lose their secular authority. The four groups, with the priests, make up the Sanhedrin, the Jewish governing body that will turn Jesus over to the Romans to be crucified. For the moment, however, Jesus wishes to warn the disciples to avoid their teaching (Matthew 16:12)—whether the love of manmade tradition or secular power.