5Then the people of Nineveh believed in God; and they called a fast and put on sackcloth from the greatest to the least of them.
6When 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.
7He 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.
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.
9"Who knows, God may turn and relent and withdraw His burning anger so that we will not perish."
10When God saw their deeds, that they turned from their wicked way, then God relented concerning the calamity which He had declared He would bring upon them. And He did not do it.
As I've said here before, the way God has crafted this book for us makes for a fascinating journey. Again, we are not given verse after verse of Scripture as to what Jonah said nor what the Ninevites' exact sins were. But we are witness to God's working in Jonah and in this people, who, obviously, were "great" to God. I really think God wants us to understand the concepts of His Word and the bigger picture. Just loving this aspect of the book!
My study Bible and some alternate translations suggest that Jonah got personal with the people and shared of his experience in the belly of the fish. Surely, crowds of fish god and goddess worshipers would listen with rapt attention in hearing of God's saving of the man before them. As for the king, he heard of God's anger with his people and the consequences He presented to Jonah. Still, for an entire city the size of Nineveh to be converted into believers, God was working not just in Jonah, but in the hearts of each and every citizen. I need a minute to let that sink in, because He is just as capable of working such a miracle today!
There's a marvelous parallel between what happens in Nineveh and what happened on the boat with the sailors.
"...Get up, call on your god. Perhaps your god will be concerned about us so that we will not perish."--Jonah 1:6, the sailors speaking
"Who can tell, God may turn and revoke His sentence against us [when we have met His terms]...."--Verse 9, Amplified Bible
For there to be true repentance, there needs to be an understanding that God is holy, and sin cannot co-exist in that relationship. One needs to understand the presence of sin, and the king of Nineveh was convicted of that. He told his people to turn away from their wickedness and violence (the Hebrew word also suggesting "wrong or unjust gain" [Strong's]). God's "terms" would be to repent and sin no more. Yet, it is God who determines who has truly repented in his heart.
Verse 10 confirms that the Ninevites were faithful in meeting God's terms. God saw what they did and knew that they had repented. In turn, He relented. I don't know how often we think about God changing His mind, but that's the idea behind 'relent'. God had a plan to bring "a calamity" upon the Ninevites. They were deserving of a calamity, just as Jonah was deserving of his consequences. Yet, in His mercy, God retreats from His plans. Under no circumstances does this say God "gave in." All along, He wanted His people to come back to Him--but it had to be on His terms, which, from this passage, happened.
Let the wicked forsake his way
And the unrighteous man his thoughts;
And let him return to the LORD,
And He will have compassion on him,
And to our God,
For He will abundantly pardon.--Isaiah 55:7
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Note: I read from the New American Standard Bible translation,
specifically, The MacArthur Study Bible (NASB).
I will quote other sources if used in a post.
I also use Strong's Exhaustive Concordance of the Bible
(with notes from the King James Version).