The religious leaders asked Jesus a question meaning to trap Him.
Then some of the scribes and Pharisees said to him, "Teacher, we want to see a sign from you."
Matthew 12:38 CSB
Give Us a Sign
The scribes and Pharisees weren't hoping to give Jesus a chance to prove Himself—they were trying to trick Him into disproving Himself. Sometimes asking questions is evidence of a growing faith, but here it was evidence of doubt and unbelief.
If these hypocrites had used their eyes with open hearts, then they would have been overwhelmed with the signs that validated Jesus's identity as the Messiah King. But they were so blinded by their narrow, selfish interests that they missed the obvious.
39 He answered them, "An evil and adulterous generation demands a sign, but no sign will be given to it except the sign of the prophet Jonah. 40 For as Jonah was in the belly of the huge fish three days and three nights, so the Son of Man will be in the heart of the earth three days and three nights. 41 The men of Nineveh will stand up at the judgment with this generation and condemn it, because they repented at Jonah's preaching; and look—something greater than Jonah is here. 42 The queen of the south will rise up at the judgment with this generation and condemn it, because she came from the ends of the earth to hear the wisdom of Solomon; and look—something greater than Solomon is here.
Matthew 12:39-42 CSB
The Sign of Jonah
Jesus's answer revealed the evil in their hearts: He had already proven Himself as Messiah, but their hard hearts prompted them to continue to test Him.
Jesus clearly saw Jonah as a book of historical fact—the way He spoke of Jonah assumes this is agreed upon.
Matthew showed Jesus as greater than the priestly cult, prophets like Jonah, and kings like David and Solomon (vv. 3-8). Yet we cannot see these truths without believing eyes and hearts.
Renewal requires transformation. Signs do not require transformation.