“As Jonah became a sign to the people of Nineveh”
For as Jonah was a sign to the Ninevites, so the Son of Man will be a sign to this generation. (Luke 11:30 BSB)
What it sounds like today
The sign of Jonah, minus the famous fish.
The cultural key
Matthew's version spells out the analogy: three days in the fish, three days in the heart of the earth, a resurrection sign. Luke leaves the fish out entirely: Jonah himself 'became a sign' to Nineveh, and the weight falls on what follows in both Gospels, that Nineveh repented at Jonah's preaching. How was Jonah a sign to people who never saw the fish? He arrived as a man delivered from certain death, carrying a summons to repent, and the pagans obeyed it.
Recovered meaning
Luke's cut makes the accent audible: the sign is not the spectacle but the preacher, back from the deep with a warning, and the scandal that Gentiles repented at less evidence than this generation is refusing. Matthew hears Easter in Jonah; Luke hears the sermon.