It is, above all, a good yarn. A pusillanimous preacher thinks he can get away with not risking his life preaching, as God had told him to, to the wicked people of Niniveh. No way ! Casting lots convinces the crew that the storm God sends to scare the heebie-jeebies out of them on the boat in which he tries to escape his prophetic duty, was caused by Jonah. So they chuck him overboard. God has second thoughts and saves his soul and his skin by getting a whale to swallow him whole. Three days and three nights in the belly of the marine monster are enough to give Jonah second thoughts too, so when he is ejected safe and sound he finally fulfils his divine mission and converts the Ninevites. The story-teller adds a few more details in a sequel about God deciding to pity the formerly naughty, now nice, Ninevites, but the message is clear : you don’t mess with the Almighty and His divine decrees.
The myth is one of the world all-time best-selling book’s most memorable stories. Did anyone ever take it literally ? Probably. Jesus, like the people listening to Him, Himself seemed to (Matt.12:38-41 and 16:4, Luke 11:29-32), in using it as an analogy of His own promised survival after three days in the tomb. As this is the central belief of Christianity, it would not make much sense comparing the Resurrection with a mere story about something that everyone knew never happened.
Credulity in 8th century B.C. Judaism, credulity in Christianity’s first century, credulity in Christianity ever since, and even, as of the 7th century A.D., in Islam. Believers will swallow just about anything.