I was sitting in my bedroom as an angst-filled 16 year old kid, looking for God to show me some sign that he knew what he was doing. My family had relocated to Miami, FL, which meant that I was starting at a new high school as a Junior. This was before the days of cell phones and text messaging, and even email and IM's were not ubiquitous, so the friends that I had done life with since Kindergarten were essentially incommunicado, and I was feeling very alone.
So I prayed, God, show me a sign. I opened my Bible, let it simply fall open, and plopped down my finger. It landed on Matthew 8:26: "Why are you afraid, O you of little faith?"
I freaked out.
Now, I don't know whether or not that was God giving me a sign. It's entirely possible that it was just coincidence, and even though it did work out to be what I needed at that time of my life, it's not a method I would recommend others use to get a sign from God! I tell it only to point out one universal truth of the Christian life: we want God to give us a sign.
Maybe it's our lack of confidence in our ability to determine God's will, or our fear of messing up, of failure, or some other such thing. Maybe were not confident in God's ability to actually come through when we really need him to. Whatever the case, we're a lot like the religious leaders in Matthew 12 who demanded that Jesus give them a sign.
Their issue was that they really didn't believe that he was who he said he was. Jesus was teaching the message of the coming kingdom of God, and performing miracles and healing wherever he went. His teaching and his behavior were proving that he considered himself to be one with God (even God himself!), and what's more, he was treating their religious texts like they were being fulfilled right before their very eyes. He was redefining what they had come to believe, claiming that what they thought God intended and what he actually intended were two different things.
So they asked him for a sign. Prove it. Prove that you're God.
It's really not that outrageous if you think about it. If I heard stories about a miracle worker near my town, my first response would be skepticism. I'd go out to see him, check him out, watch the first few hearings, and in my skepticism I'd assume that they were all "plants". People the healer had brought up who weren't really sick but were just pretending so that it looked like he actually healed them. If I had a chance to talk to him–and especially if he were redefining the rules of Christianity and pretending to be God–it might be the first thing I'd ask him: can you perform an on-demand miracle?
If he's really God, an on-demand miracle should not really be that big of a deal.
Jesus doesn't give it to them, though. What he says instead is that they won't receive a sign–except for one. They'll receive the sign of Jonah. According to Jesus, that sign will be so powerful that these religious leaders will look like fools if they reject it; indeed, if they reject the obvious proof of the sign of Jonah, then they are worse than the Ninevites who repented after hearing Jonah's message. "And now", Jesus says, "one greater than Jonah is here."
The "sign of Jonah" is the three days Jonah spent in the belly of the fish, presumably a dead man. Jesus was going to spend three days in the belly of the earth, definitely a dead man. On the third day, Jonah was vomited out onto dry land. On the third day, Jesus was going to be "vomited" out of the grave; death itself would be reversed and Jesus would be the "firstborn" among many children of God.
For those demanding a sign, Jesus points towards his death and resurrection and says, "that's enough". You don't need on demand miracles. You don't need more proof than you already have to know that I am God. Look at what I accomplished for you.
Jonah's story is about God's constant pursuit of his rebellious people; Jesus' story is about God's constant pursuit of his rebellious people. Jonah ignored God and needed to be pursued himself; Jesus obeyed and became the pursuer ("The son of man came to seek and save the lost.") Jesus pursued his people, even to death itself, and then, in death, sets them free.
It would be nice if God gave us a sign to tell us what to do on a daily basis. Maybe he will, at times. But he doesn't have to. He's already given us a sign to demonstrate that he will not let us go, he will not stop pursuing us, he will not leave us alone, ever again.
The sign of Jonah.
And that's all we need.