Jonah 3:And (Jonah) cried out (to the people of Nineveh), “Forty days more, and Nineveh shall be overthrown!” 5 And the people of Nineveh believed God; they proclaimed a fast, and everyone, great and small, put on sackcloth…10 When God saw what they did, how they turned from their evil ways, God changed his mind about the calamity that he had said he would bring upon them; and he did not do it… 4 But this was very displeasing to Jonah, and he became angry. 2 He prayed to the LORD and said, “O LORD! Is not this what I said while I was still in my own country? That is why I fled to Tarshish at the beginning; for I knew that you are a gracious God and merciful, slow to anger, and abounding in steadfast love, and ready to relent from punishing. 

God had called the Jewish prophet Jonah to warn the Ninevites (an enemy of Israel) of God’s coming judgment. Jonah, suspecting God might have mercy on the Ninevites instead of destroying them, fled in the opposite direction. You’ve probably heard how Jonah was tossed overboard on a ship then was swallowed by a large fish (commonly referred to as a whale) which spit him onto land after three days. Apparently Jonah would not be allowed to refuse God’s call. So Jonah proceeded to Nineveh, which is where our passage picks up the story.

Seeing the repentance of the Ninevites God changed his mind and had mercy on them.

This is exactly what Jonah had feared. It upset him that God had mercy on the bad people of Nineveh. I suspect we will be surprised like Jonah when the day of judgment comes and we see the persons who are saved from destruction and who are not. I expect there will be “bad people” who repent and are saved and “good people” who trust in their own righteousness to their peril.