The only sign you will be given is the sign of Jonah. The crowd’s demand of a sign from Jesus (to show that he was not working for Beelzebul) did not augur well with Jesus. It was an indication of the slowness with which they were warming towards him. At worst, it was a slap on his face for it demonstrated not only their indifference to those who were suffering various illnesses but also their ingratitude to Jesus. It was a demand that totally amazed Jesus. What more sign other than what he was already doing could the people possibly want? Wasn't his very life sign enough? And if this wasn't enough, what about the miracles he had been performing? Wasn't raising the dead sign enough? What about feeding the hungry or restoring wholeness to those who were suffering all kinds of sicknesses? Apparently, the crowd didn’t think highly of these. Jesus was not going to fall into their trap. However, he promised them one thing: the sign of Jonah.
Whereas the one sign that readily comes to mind whenever the name Jonah is pronounced is that of Jonah spending three nights inside the whale, there is another sign that is just as important, even though it is not as explicit as Jonah spending three nights inside a whale. The sign that Jesus was promising the crowds is the coming to repentance of the people of Nineveh. No one, not even prophet Jonah himself, expected the Ninevites to repent. Their sins were so numerous, and the distance between them and God was, according to the standards of the time, irreconcilable. But to the surprise of Jonah (and God too), the Ninevites repented at the preaching of Jonah. It was something that could only be explained as a miracle (sign), as something made possible from the heavenly power.
It is God who bridges the distance that sin creates between man and God. The people of Jesus' time believed that such a thing was not possible (why they accused Jesus of blasphemy whenever Jesus told those he had healed that their sins were forgiven). The great sign that those who were slow to believe in Jesus would see is the coming to believe in God by the peoples of the nations. No sign would ever surpass this: to see a people hitherto considered unworthy of God coming to believe in the one God of the Jews. In Jesus Christ, God was going to make possible what has been thought to be impossible. It will be something that shows a face of God that was thought to be non-existent: a forgiving, merciful, and repentant God (the same God who had repented of the plan to annihilate the Ninevites).