No sign will be given them except the sign of Jonah. The passage constituting today’s Gospel Reading is a follow-up to an earlier incident in which Jesus was asked for a sign as proof that he was from God, or rather, that the signs he was performing were sanctioned by God (cf. Luke 11:14-22). As would be expected, Jesus got offended by the request/demand. What more sign did they want other than his life-giving presence, the greatest sign that has ever been (greater than Solomon the wise)? God was not going to bow to their request because it was due to their stubbornness of heart that they had closed their eyes to the greatest sign that was living in their midst. The only sign that the Lord was going to give them was the sign of prophet Jonah. How was Jonah a sign? 
When God commanded the prophet Jonah to bring God’s message of repentance to the inhabitants of the pagan city of Nineveh, Jonah naturally objected. Not only was Nineveh an enemy of Israel, Jonah surmised that it would also be a total waste of his time. Nineveh was a wicked city whose citizens had never been known to repent or follow God's command. When God insisted that the mission had to be carried out, Jonah still didn’t budge. He was so much opposed to the idea that he even offered to denounce his call as a prophet. However, God was not going to let him do that. When all his attempts to run away from both God and the mission failed, Jonah reluctantly gave in. He went through the city of Nineveh and made the Ninevites aware of the impending doom which he was sure would come upon them. To Jonah’s surprise, the Ninevites, led by their king, repented of their sins. The sight of remorseful Ninevites covered in sack clothes and ashes prompted God to repent from his plan to annihilate the Ninevites. But while God was glad that the Ninevites had come to heed God’s message, the same could not be said of the prophet Jonah. He was bitter that God had spared the lives of the Ninevites. He even asked God to end his life. He reasoned that it was better for him to die than see the Ninevites escape annihilation. How could God's prophet, a messenger of God, become such a sadist? Jonah did not believe that the Ninevites could change. He did not believe that God could show favor to a pagan people. 
To the people who were demanding for a sign, Jesus proposed to them the sign of Jonah. He gave them the story of Jonah and the Ninevites as a sign of things to come (or rather, of the things that were already taking place). If Jonah was surprised at the sudden coming to repentance of the Ninevites, so should those who were demanding for a sign get themselves ready to be surprised. They were to get ready to witness bigger things than what had happened during Jonah’s time. God, through Jesus, was going to make disciples of all nations, including those who had been thought to be eternally unworthy of God. Men and women who were thought to be unredeemable were going to accept Jesus as Lord and become members of God's household. The people who had always been regarded as the least favored by God were going to become the recipients of God's favor. For unlike those who were gathered about Jesus who took their belonging to God's family for granted so much so that they turned a deaf ear to God's words, the people of the nations would heed God's word and come to God. It would indeed be a great sign since instead of only the Ninevites, all the nations of the earth would repent and come to believe in God.