“The people of Nineveh
Believed God.
They proclaimed a fast.
Everyone,
Great and small,
Put on sackcloth.”
In a sudden conversion or the fear of destruction, the people of Nineveh believed in God. Notice that they believed in God, not Yahweh, which would have been more difficult. Just like on the boat, Jonah seemed to be successful, even if he was not trying very hard. They proclaimed a fast. Then everyone, no matter whether they were important or not, put on sackcloth to go into mourning.
Eugene Finnegan Bible Project 