This fits with the various warnings Jesus gave about leavening. He warned about the leaven of the Pharisees, the leaven of the Sadducees, and the leaven of Herod. In each case, leavening is a symbol for corrupted ideas that lead to a wrong application.
In Matthew 16:12, we find that part of the leaven of the Pharisees was their doctrines, which elevated tradition above the word of God. Another part of their leaven was hypocrisy. They focused more on appearing righteous than being righteous. Jesus said of the Pharisees, “they say, and do not do” (Matthew 23:3). They exempted themselves from the standard they held others to. While they looked good, their self-centeredness and self-generated standard of righteousness made life very difficult for those caught in their orbit.
The leaven of the Sadducees was their corrupted teachings and attitudes, like skepticism, that undermined the essential doctrines of faith. As two examples, the Sadducees denied the resurrection and they denied the existence of angels. That is a significant corruption.
