No sign will be given them except the sign of Jonah. Our Gospel account today 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 was sanctioned by God. And of course, Jesus got offended by the request. What more sign did they want other than his 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 their stubbornness that had blinded them from seeing the greatest sign that was living in their midst.
The only sign given you will be the sign of Jonah. What about Jonah? Jonah was sent to preach to the Ninevites, and naturally he objected because he knew that it would be a waste of his time: they had never been known to repent or follow God's command. Jonah even tried to denounce his call but God would not let him. To his surprise, the Ninevites repented and God repented from the plan of annihilating the Ninevites. And this angers Jonah to the point of asking God to end his life. 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.
Jesus is giving the people Jonah as a sign of things to come. If Jonah was surprised, so should they get ready too. They should be ready to witness bigger things than Jonah did. God, through Jesus, was going to make disciples of all nations. 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 they have always regarded as the least favored by God are going to be 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 deaf ears to God's word, the people of the nations would heed God's word and come to God. It would indeed be a great sign for this time rather than only the Ninevites, all the nations would repent and believe in God.