From Luther’s lectures on Jonah in 1526, here discussing chapter 4, verses 1 and 2:
That Jonah is sent from the land of the Jews into a foreign country symbolizes that the Spirit and God’s Word were to be taken from the Jewish people and bestowed on the Gentiles. Thus Christ says in Matt. 21:43: “Therefore I tell you, the kingdom of God will be taken away from you and given to a nation producing the fruits of it, etc.” Jonah’s flight and the perils he encountered on the sea represent the cross and the persecution which the Gospel will experience in the world. It appears as though the Christian ministry were taking to flight, perishing, and vanishing from the scene. It seems so frail over against the fury of the world, since the persons administering the office are fugitives, that is, feeble and insignificant people. The ocean is the world, vast and mighty with its raging billows. The whale is gruesome and terrifying with its jaws and teeth. This animal represents the prince and god of the world, the devil, who ruthlessly murders and kills through his princes and great lords, etc. But despite all of these, Jonah is preserved mightily by God’s power, and his message cannot be frustrated either by his own flight or by the ocean’s fury, but it makes its way and penetrates into Nineveh. Thus, though pastors may be weak and the world powerful, God’s Word, the holy Gospel, is still mightier, and no obstacles can impede its progress. And even if all pastors were to be devoured, the Gospel will make its way into the world all the better and transform the world. It is consoling for us to observe that this was the experience of the apostles. And we, too, must not be terrified by ocean and whale, convinced that our Word or Gospel is mightier than all else.
Martin Luther, Luther's Works, Vol. 19: Minor Prophets II: Jonah and Habakkuk, ed. Jaroslav Jan Pelikan, Hilton C. Oswald and Helmut T. Lehmann, Luther's Works (Saint Louis: Concordia Publishing House, 1999, c1974). 19:97.