In Matthew 12:39, Yeshua is recorded as saying to the gathered Pharisees, "...An evil and adulterous generation seeks for a sign, but there shall be given no sign to it, except the sign of the prophet Jonah."
Many have understood this verse to be an unequivocal claim to "messiahship" made by Yeshua. That Yeshua was saying that Jonah was a “sign” to his generation even as the he, Yeshua would be to his. However, understood in context, the verse is a simple reference to an old testament event with definite meaning. Its meaning is complicated by the following verse (40), "For as Jonah was three days and three nights in the whale's belly: so shall the Son of man be three days and three nights in the heart of the earth."
First of all, verse 40 begins in a way ( "ὥσπερ γὰρ", "just as") that implies that it is a clarifying (or parenthetical) comment of the gospel writer, and not Yeshua's; . Also, it refers to an event that was yet to happen at the time Yeshua was speaking. These two facts lend support to the notion that it is not original to Yeshua. It also indicates that the Gospel writer (or more likely translator) misunderstood the reference to Jonah as well.
Yeshua's reference to the "sign of Jonah" would have been well-understood by those listening. In order to comprehend it, we must look at the story of Jonah:
Jonah was a prophet who was ordered by God to go to the city of Nineveh to prophesy against it "for their great wickedness is come up before me" Jonah instead flees from "the presence of the Lord" by going to Jaffa and sailing to Tarshish. He is then tossed overboard and famously swallowed by a fish. This diversion has hindered the interpretation of Jonah's story for centuries.
Jonah fled from God, not because he feared the Ninevites, but because he disagreed with God's plan for them. Nineveh was a wicked city on a caravan road at the banks of the Tigres river. Jonah knew that the Ninivites had invited God's wrath and were deserving of nothing short of destruction. Jonah also knew that the only chance that the Ninevites had of survival was if God sent them a warning and that they subsequently heeded that warning. Jonah had predicted the outcome; namely that he would prophecy to the Ninevites, and that they would repent and be saved. This was the last thing that Jonah wanted, and, although it was God's will, Jonah tried to flee to avoid its implementation.
Eventually, Jonah went to Nineveh and gave Yahweh's message to the people there. Jonah then "went out and sat down at a place east of the city, and there he made himself a shelter, sat in its shade and waited to see what would happen to the city." Predictably, the city repented and it was spared. This angered Jonah and He said to God, "...is this not what I said when I was still at home? That is why I was so quick to flee to Tarshish.... Now, O Yahweh, take away my life, for it is better for me to die than to live." This is when the "sign of Jonah" or more appropriately, "Jonah's sign" appeared.
Jonah 5:6-7 tells us that: "Yahweh God provided a vine and made it grow up over Jonah to give shade for his head to ease his discomfort, and Jonah was very happy about the vine. But, at dawn the next day God provided a worm, which chewed the vine so that it withered." God explained to Jonah, "Do you have a right to be angry about the vine?" "I do," Jonah replied.
But Yehweh said, "You have been concerned about this vine, though you did not tend it or make it grow. It sprang up overnight and died overnight. But, Nineveh has more than a hundred and twenty thousand people who cannot tell their right hand from their left, and many cattle as well. Should I not be concerned about that great city?"
So, "Jonah's sign," is the vine, which in Hebrew idiom represents Israel. God is chastising Jonah for being concerned only for the fate of the vine (Israel) and not the fate of the great city (representing all of the nations), which is much more extensive.
Looking back to Matthew, the writer, at verse 18, had quoted Isaiah saying, "In his name the gentiles will put their hope." This is an astounding statement. When Yeshua later healed a man (probably a Roman given the context), some of the Pharisees accused him of being Beelzebub (a gentile pagan god). Others were not sure, and Matthew tells us that some of them asked to see a miraculous sign. Recall that they had just witnessed him healing a man. So, what did they want to see? Presumably, they were asking Yeshua to show a sign, which was undeniably from Yahweh. In other words prove that he was a Jewish prophet of the Jewish God, Yahweh and not Beelzebub or some other God of the Gentiles (nations). Yeshua responded, "none will be given it except the sign of the prophet Jonah."
In other words, Yeshua was saying, (paraphrase), "Why are you concerned about Jews only?" Like the vine of Jonah, the Pharisees did nothing to create Israel or to "make it grow." Like Niniveh, the numbers of Gentiles are great. Should Yeshua not be concerned about them?
Astoundingly, Yeshua was declaring himself as the hope of all mankind, not just the Jews. Just as Yahweh demonstrated to Jonah at Niniveh that he was the hope of all mankind.