God saw what they were doing—that they had ceased their evil behavior. So God stopped planning to destroy them, and he didn’t do it. Jonah 3:10
The Ninevites made real changes and then God made a change. He changed his mind. God stopped the plans to destroy Nineveh. There was nothing in God's word that Jonah declared that was conditional. God did not say, "if you repent, I will relent." I read the word as saying, "it's over, but I'm giving you forty days."
The king on Nineveh imagined out loud when he said, "who knows? God may see this and turn from his wrath, so that we might not perish." Having not been schooled in how prophecy works and in the face of ruin, this king of a people who have a track record of evil is suddenly optimistic; while at the same time, being gravely certain of the truth of Jonah's word and leading his people in sincere repentance.
But why 40 days? Even the 40 day warning was merciful. The question would be, "what would you do if you found out you only had forty days to live?" The Ninevites, in whole, decided to spend the 40 days repenting, calling out to God, and stopping their evil behavior.
This story illustrates the fact that all prophetic words are not binding. A negative word that says, bad things are going to happen to you, even death, is perhaps God saying, "if you continue on the road you are on, you are going to get where you are going." If that person responds to that word by getting off the road they are on or turning back onto God's road, and they do not die, but live in God; that does not make God a liar or the prophetic person false. They were actually very accurate from Heaven's perspective. The word's purpose was to wake up the sleeper. Rude awakening precedes great awakening.
The opposite kind of prophecy is the positive one. The prophetic word foretells blessings in the person's life that are not presently evident. After receiving the positive word, the person's circumstances may get worse, perhaps for a long time. Was it a false prophecy? No. The word was meant to encourage the person about their future and give them hope. Is the positive word conditional? Sometimes it is. The person may have to walk with God, holding onto their word, through a desert; where they learn to trust God's word above all else, before their word of promise is fulfilled.
God's word to the Ninevites was true and said what was going to happen. But then, God changed his mind. God changed his mind because of the new evidence set before him. But being God, wouldn't God have known what the Ninevites were going to do and that he would not be destroying them? Either he was playing a game where the ends justified the means, or he really is not as smart (all knowing) as we thought, but is like a super computer or a super man.
No, no, no. None of these are correct. God is almighty. But what if God in his infinite wisdom, chooses not to know certain things? God is in control, but not controlling. What if God hopes for an outcome, but he does not know until we make our choice? It really is not a rigged game. What if God is like a parent that disciplines his child. Like the parent, God hopes the discipline will lead to obedience and reconciliation. God is a person. We were created in God's image.
What if God, since he is God, can know everything, but chooses to not know some things?
An intriguing example of this is in Genesis 22, when God sends Abraham up the mountain to sacrifice his precious son, Issac. The story opens by telling the reader that God tested Abraham. OK, it's a test, a really intense, hard test; that Abraham passed. Why did God do that to his friend? Abraham passes the test. He was about to kill his son and then God, through the Angel of the Lord, said, "Don't stretch out your hand against the young man and don't do anything to him, I now know that you revere God and didn’t hold back your son, your only son, from me.”
Notice the Angel of the Lord, speaking for God says, "I now know". In other words, he did not know before the test and needed to find out, so he tested Abraham. God didn't know. God didn't choose to know, for some reason in God's infinite wisdom. It doesn't say, "I knew (I'm God), but I wanted to see what I already knew". But it says, "now I know", implying that God did not know.
Back to Nineveh. God knew what could happen, for sure. But God may have been choosing to not know what would happen, but was truthfully declaring, through Jonah, that destruction was in forty days. As God, God had decided it was curtains time for Nineveh. Enough was enough.
But when Nineveh changed, God changed. God who is a rock and changes not changed. God's character did not change, but God's mind changed. God changed his plans after seeing what the people of Nineveh had changed about their behavior. God, who is the almighty and the righteous judge also is merciful.