The only sign you will be given is the sign of Jonah. After refusing to accept the exorcisms of Jesus as authentic, the scribes and the Pharisees continued to display their disapproval of Jesus by demanding a sign from him. It was a demand that could as well have been understood to be an effort by the religious leaders to have Jesus prove once and for all that he is truly from God (or rather that what he was doing was sanctioned by God). However, Jesus refused to cede to their request because he deemed it an insult. What more sign could they possibly have wanted? Wasn't his very life sign enough? And in case his life wasn't enough, what about the miracles he had performed? Wasn't raising the dead sign enough? If what they had witnessed by their own eyes was not enough to convince them, then there was nothing more that Jesus could do to have them change their minds. He was not going to give them a sign, at least not the type they expected. But he promised them something: the sign of Jonah. It was a sign that they were to be given, not for the satisfaction of their curiosity, but rather for their reflection.
While the one sign of Jonah that readily comes to mind is the prophet spending three nights inside the belly of the whale, the sign that Jesus was promising the religious leaders is the coming to repentance of the people of Nineveh. No one, not even prophet Jonah himself, had expected the Ninevites to repent. Perhaps in the eyes of the likes of prophet Jonah, the sins of the Ninevites were just too numerous. Moreover, as a people who didn’t have any regard for the ways of God, the distance between them and God was thought to be irreconcilable. However, to everyone’s shock, at the preaching of Jonah, the Ninevites did repent of their wayward ways. It was an unexpected turn of events. The only ‘logical’ way to explain it is to see it as a miracle (sign).
According to Jesus, because of the hardness of heart of the Pharisees and scribes, the only sign that they needed was seeing the gentiles coming to believe in God. No sign would ever be greater than that of a people who had been considered pagan and ungodly coming to believe in the one God of the Jews. In the coming to belief of the gentiles, God would make possible what has been thought to be impossible. It will be something that shows a face of God that was thought not to exist: a forgiving, merciful, and repentant God (the same God who had repented of the plan to annihilate the Ninevites). And there is no greater sign than this.