3:5 - Then the people of Nineveh believed in God; and they called a fast and put on sackcloth from the greatest to the least of them.
Here is a classic paragraph that is written in the Eastern style: the summary is given first and the expanding/explaining is given in the subsequent verses. Verses 6-9 explain in greater detail just how the events of verse 5 happened.
So, in summary, God's message is received and acted upon by the people of Nineveh, much like the sailors responded to God's message in the storm.
Here is a good place to note what God may have used to soften the Ninevites' hearts to His message:
1. Jonah's appearance. Having been floating in the acidic stomach of a fish, Jonah may have looked like death itself. His skin would probably have been bleached and raw as well as his face, hair and clothes.
2. John MacArthur notes in his study bible that Nineveh also experienced two plagues and a solar eclipse prior to the events of Jonah, which would further prepare Nineveh for Jonah's message.
3:6 - When the word reached the king of Nineveh, he arose from his throne, laid aside his robe from him, covered himself with sackcloth and sat on the ashes.
In one day, the word had reached the entire city and sparked a revival like no other. From the king to the commoner, God produced repentance from Jonah's message. Jonah does the bare minimum, and God does the rest.
The king demonstrates his own repentance by getting off his throne (symbol of authority), laying aside his robe (symbol of comfort), covering himself with sackcloth (symbol of mourning), and sitting on ashes (symbol of repentance).
3:7 - He issued a proclamation and it said, “In Nineveh by the decree of the king and his nobles: Do not let man, beast, herd, or flock taste a thing. Do not let them eat or drink water.
The king issues a command to the whole city (with the weight of the throne behind it) to fast. Including the animals in the fast was a common custom in Persia for mourning ceremonies, according to MacArthur.
3:8 - But both man and beast must be covered with sackcloth; and let men call on God earnestly that each may turn from his wicked way and from the violence which is in his hands.
The command of the king continues with direction to don sackcloth, call on God, and repent from their wicked way. It is interesting to note that the city of Nineveh responds to God's second call to repentance - the general call of plagues and eclipse did not achieve the desired response, but the confrontation by one does.
Another interesting thing found here is that while Jonah's message mentions absolutely nothing about the sins/wickedness of the people of Nineveh, the king and the people knew their sin - "turn from his wicked way and from the violence which is in his hands". They also knew that they deserved judgment.
Note that Assyria had a reputation for brutality, and Nineveh was the worst of the country. Some of the atrocities committed by the Assyrians included flaying people and covering walls or pillars with their skin, building pillars and walls from heads or corpses, burning captives with fire, cutting off body parts of captives, and mounting heads on pikes around the city. Nineveh was steeped in an absolutely evil and wicked culture.
3:9 - Who knows, God may turn and relent and withdraw His burning anger so that we will not perish.”
The fear of the Lord has come to Nineveh. The king recognizes the truth of the situation and motivates his people to repent, by God's grace and direction. While the sailors responded to the will of the Lord by initially fighting against it, Nineveh responds immediately in bowing to His will in repentance.
It is interesting that the Ninevites repent with no promise of mercy from the Lord. Perhaps they recognized that since they were given a warning that they could obtain mercy from the same God that gave them the warning.
Here is the climax of the second half of the book and it is similar to the climax of the first half. The apex is once again a form of a question: "Who knows, God may turn and relent and withdraw His burning anger so that we will not perish.”
While the king and nobles command the signs of repentance: fasting, sackcloth, and prayer, it appears that everyone in Nineveh came to true repentance as we will see the effect in the next verse.