NOTES for Mar 8:11-21
Jesus calls the demand for signs and miracles the "leaven of the Pharisees and Sadducees." This is often understood as a reproach for hypocrisy, for insincerity, and in some cases insincerity and hypocrisy really were present. Not always, however, was the issue conscious hypocrisy as we understand it today. Both the Sadducees and the Pharisees were, first of all, religious people. The Sadducees were somewhat more liberal; they recognized only the Pentateuch as sacred text, they had no developed teaching about resurrection and no clear messianic concept, but in everything else they were still orthodox Yahwists, including in matters of practical observance of the Torah.
The Pharisees, in addition to the Pentateuch, also recognized the prophetic books as sacred; for them Holy Scripture was "the Law and the Prophets." They had a clear messianic concept and a more or less developed teaching about the general resurrection. Yet despite all their disagreements with the Sadducees, the Pharisees still did not consider them heterodox, especially since the temple priestly leadership mostly held precisely Sadducean views. What united the Sadducees and the Pharisees was exactly that both groups were religious people: some more sincerely and firmly, others less so, and some purely formally, as always happens in a religious environment. This is precisely why the Pharisees demanded signs: they needed to test Jesus and everything He does for conformity to their messianic concepts. To fit Him into their religious system.
But He was not going to fit into any system, not, of course, in order to annoy religious people, but because He simply could not fit into it. The Kingdom does not fit into any messianic concepts. It is greater than any system a person could invent, greater than anything the human imagination could picture. And every revelation in general, even a less sweeping one, is always greater than any religious system a person could devise.
Greater even not in the sense of scale, though in this respect as well revelation usually surpasses the human mind, but in the sense of that spiritual dynamic, that life, in comparison with which even a brilliant scheme created by a human being will be no more than a static cross-section. To fit living spiritual life into this flat, two-dimensional cross-section is completely impossible; at best, one can only hint at it. But religious consciousness stubbornly tries to reduce life as such to it. This striving, this logic, is what Jesus calls the "leaven of the Pharisees and Sadducees," warning His disciples against it.