The book of Jonah is gloriously full of gospel. The book of Jonah is also, to a certain degree at least, about a whale (or a big fish). I believe in the whale. Or rather, I should really say, I believe in the historicity of the whale (and if you make it to the end of the post I'll explain that change in wording).
Not every evangelical believes in the historicity of the whale. Some prefer to read the book of Jonah as a parable. But let me give you five reasons why I think it should be read as history rather than parable. (And then let me explain one other thing.)
1) Jonah is identified
Jonah is named. Think of Jesus' parables for a moment. In all the parables there is only one person who is named - Lazarus (he of the rich man and Lazarus fame, not the one who was raised from the dead). In the other parables there is a man, a sower, a king, a woman, etc., but none of them are named. (In fact, the very fact that Lazarus is named has even caused some interpreters in the history of the church to question if he really was a parable.)
Now, those who favour reading Jonah as a parable will point to the fact that the king isn't named as evidence in favour of their interpretation. But, this wouldn't be the only historical narrative in Scripture which omits the name of the Gentile king. After all, who was the Pharaoh who favoured Joseph, or the Pharaoh whose daughter found Moses, or the Pharaoh of the Exodus. While those may be the sort of questions to which Old Testament scholars, historians and archaeologists can suggest answers, they are not questions to which Scripture gives us an explicit answer.
And not only is Jonah named, but he's identified even more precisely than that, because we're told his father's name as well. That means he's identified as a specific individual; his name is not merely a symbol.
2) Jonah spoken of by Jesus.
Jesus knew about Jonah and spoke about Jonah. And Jesus didn't refer to a parable of Jonah, but to “the prophet Jonah” (Matt. 12:39; 16:4; cf. Luke 11:29). Jesus identifies Jonah as one of the prophets, and so, like all the other prophets, as a real person.
In the same way, Jesus refers to the Ninevites to whom Jonah preached as real people (Matt. 12:41; Luke 11:32), otherwise how could they rise up on the last day?
And sandwiched between Jesus speaking of Jonah as a real person and speaking of the Ninevites as real people, there's the whale (Matt. 12:40).
3) Nature of the sign - life from death
The reason Jesus speaks about Jonah is to point to “the sign of the prophet Jonah” (Matt. 12:39). What Jesus tells us is that Jonah's three days in the belly of the whale points to Jesus' three days in the tomb.
And this is where most objections to the historicity of Jonah seem to crop up. The three days in the belly of the whale seems to be the crux of the matter. Some would argue that it would be impossible to survive three days in the belly of a whale (or large fish), and therefore it must all be a parable.
But, that's exactly the point of the sign. The sign is about life coming out of death. Jonah should have died, but God bring's him out of what should have been a tomb alive. And as a result salvation flows to the Gentiles.
To be honest, I'm not at all interested in working out how long Jonah could have stayed alive inside the whale — that misses the point entirely. The point is that God brings life out of death. It's a miracle! The point is that God works a miracle so that Jonah's three days in that living tomb, followed by life as if from the dead, functions as a sign pointing us to Christ, who spent three days in the tomb before rising again to life from the dead.
4) Jonah mentioned elsewhere in OT.
Jonah isn't the only Old Testament book that mentions Jonah. The same Jonah (same name, same father, same job) appears in 2 Kings 14:25, where we also learn where he was from and when he lived (during the reign of Jeroboam II, a date which explains the use of “king of Nineveh” rather than “king of Assyria”).
5) Specific places
Finally, the book of Jonah mentions specific places. Tarshish, Joppa and Nineveh were real places, and significant ones too. (Although there is some dispute over the exact location of Tarshish, it is clear from both the Bible and Assyrian records that it was both a real and a significant city.) As with people's names, so with place names — they tend not to be mentioned in parables. So real places again suggests real history.
One more thing
So, there are my five reasons for believing the historicity of Jonah and the whale. But now, let me add one other thing. I want to say “I believe in the whale”, but instead I say, I believe the whale really existed; I believe that the book of Jonah is indeed an historical account. So why not simply say, “I believe in the whale”?
Well, I have a friend to blame/thank for that. When asked once if he believed in the whale, he pointed out that he believed “in God the Father Almighty, maker of heaven and earth and in Jesus Christ his only Son our Lord”. You see, there's a big difference between what we believe exists/existed and what we believe “in”. When we say we believe in Jesus, we're talking about saving faith that unites us to Christ. And, no matter how passionate one may be about the historicity of Jonah and the whale, believing in the whale is not saving faith. So, I believe that the whale existed, but I believe in Christ.
Also, that means that believing the historicity of the whale is not the be all and end all. Lack of belief in the historicity of the whale is not the same as lack of faith in Christ. The whale may point us to Jesus, but Jesus is not the whale! As the Old Princeton theologians, Hodge and Warfield, pointed out when they set out the case for the doctrine of inerrancy, inerrancy is not the foundation of the Christian faith. (And yes, I know some would claim to both uphold inerrancy and think Jonah's a parable.) It's not inerrancy (important as it is) that makes Christianity true, but Jesus!
So while I believe that the whale existed, I'm not going to panic about the salvation of my friend who doesn't. It's Jesus who saves, not the whale. So our confidence must be in Jesus alone.