We cannot know in advance what God has decreed, but we do know the conditions for blessing and cursing He has revealed in His Word. We should pay heed to those conditions, knowing that if we trust in Him, we will be blessed forever, but if we reject Him, we will be cast out of His gracious presence for eternity.
Without question, God knew that the Ninevites were going to repent long before He sent Jonah to preach to them. In fact, because the Lord has ordained even our responses, He always knows how we are going to respond when He delivers His Word. But God moves us to make these responses by revealing His character to us and then announcing judgment or blessing. In so doing, He creates a real relationship with His people.