The Pharisees appear once more to challenge Jesus by requesting a sign, some demonstration that he is more than just a Zealot rabble-rouser. He proclaims that there will be no sign forthcoming for his generation. In the gospel of Mark, that is the end of the discussion. No sign. Period. This, despite the claim that he was going about the countryside healing people left and right and feeding multitudes with miniscule amounts of food. So apparently these were not signs of anything spectacular. Most likely, this is because there were several so-called miracle workers running around in the first century. What exactly could the Pharisees have wanted if healing and wonder-working were commonplace enough to be inadmissible as evidence of divinity? That's not really the important question. No one can know what the Pharisees were really demanding or even if they ever actually confronted a historical Jesus. The question is placed by the author of Mark into the mouths of theoretical critics in order to prompt a theological answer: No signs for you.
In other words, according to the author of Mark, there is no proof and there will be no proof (for that generation at least) that Jesus is anything special. This answer wasn't good enough for the author of Matthew, who added the caveat,
except the sign of the prophet Jonah. For as Jonah was three days and three nights in the belly of the whale, so will the Son of man be three days and three nights in the heart of the earth. The men of Nin'eveh will arise at the judgment with this generation and condemn it; for they repented at the preaching of Jonah, and behold, something greater than Jonah is here. The queen of the South will arise at the judgment with this generation and condemn it; for she came from the ends of the earth to hear the wisdom of Solomon, and behold, something greater than Solomon is here. (Mt 12:39-42)This "sign of Jonah" was also cited by the author of Luke (Lk 11:29-32), except that the gospel of Luke doesn't interpret this sign as an allusion to death and resurrection. Actually, one might assume that resurrection would have been a pretty serious sign of something to that generation. So, somewhere between the writing of the gospel of Mark and the writing of the gospel of Matthew, belief in a messianic resurrection must have become more widespread. The gospel of Mark doesn't even include any resurrection appearances after the passion narrative. This suggests that the beliefs and assertions of the Jesus cult were still evolving as the Bible was being written.
What good is it for a person to go around claiming to be a divine figure and then assert that there won't be any forthcoming evidence? As a theological assertion, the author of Mark may have been pointing to the humility of Jesus. Presumably, he is not out to draw attention to himself through performing miracles on demand, he is doing what he is doing for a different, more noble purpose. Perhaps the author of Mark was pointing to the ignorance and blindness of the Pharisees; the verse is followed by a strongly-worded criticism against that sect. There is also a deeper message here that rings true with human experience, though. If one wants to know a person's values or deeply held beliefs, one need only observe their actions. By corollary, then, if you want people to know what you believe and value, make sure your actions demonstrate those beliefs and values clearly.
The criticism the author of Mark has Jesus level at the Pharisees actually supports this truth as well. The author of Mark doesn't spell it out as clearly as the authors of Matthew and Luke, but the general warning is the same: Beware the yeast of the Pharisees and the Herodians. (The Herodians were another political cult in the first century, supportive of Herod the Great and his dynasty.) The passage in the gospel of Mark doesn't state why there was something amiss with the teachings of the Pharisees, but the author of Luke says it plainly when he relates this teaching: The Pharisees are hypocrites. In other words, they say one thing and do another. They claim to believe something that is not clearly demonstrated in their actions. And presumably following their teachings--allowing their "yeast" to bud and ferment in one's psyche and spirit--will lead to hypocrisy in one's own life.
There is a comparison happening in these parallel passages between signs and truth. People can be so impressed with astounding behaviors and unusual events that they can go chasing after personalities with only a shallow understanding of what that individual stands for. Jonah was a "sign" to the people of Nineveh not because he was vomited up on the beach after three days inside a big fish, but because he spoke boldly, passionately, and sincerely to them. The people of Nineveh still had to take personal responsibility for what they would do with Jonah's words, and that required them to weigh his message carefully and thoughtfully. They weren't just pursuing some new shiny distraction.
What the authors of the gospel suggested about Jesus was that he was not just a shiny distraction or a bit of entertainment or even a conquering general to drive back the Romans and reclaim Jerusalem. Perhaps Jesus represented a way of being that was more about integrity than it was about miracles, more about treating people with respect than about distracting people from their lives, more about growing people through empowering them than about growing a following by bedazzling them. In other words, the core of the message is about what ordinary people can do in relation to one another, not what people must rely on an extraordinary person to do for them.
That is not necessarily the Jesus message that is propagated throughout Christian circles today, but perhaps the quality of Christlike-ness is more about living with integrity in one's own life than it is about telling others how they must live. Perhaps our human social constructs have given us permission to practice hypocrisy, and that practice has spread like yeast through our habits, our relationships, and our ideals. Whatever we may believe about Jesus, it seems that at least some portion of the gospel message advocates understanding one's own self deeply, knowing and trusting in one's guiding principles and living by them with integrity, and actually valuing one's values enough to act in alignment with them. We do not need faith for that. We do not need miraculous proof. In fact, if we were to live like this--with intentionality and integrity in all our moments--we would be the sign, perhaps even enough of a sign to inspire others. That's worthwhile leavening.