The answer is: They repented

Jonah 3:5-8 - They believed that the word Jonah spake came from God not from man and repented.

Jonah’s adventure is rich with contrast. While the prophet initially attempts to evade delivering God's message to the notoriously wicked city of Nineveh, once he finally complies, the outcome is nothing short of miraculous. The immediate reaction of the Ninevites—believing Jonah’s warning and embracing repentance—highlights a significant aspect of the narrative: the power of a sincere response to a call for change.

What’s fascinating about this story is not just Nineveh’s repentance, but the dynamics of a city-wide transformation. Picture a metropolis, filled with all sorts of characters loafing about their everyday business. As soon as word spreads about Jonah’s strident warning, from kings to common folk, they unite in humility—donning sackcloth and fasting, a sign of deep remorse and supplication. Indeed, the king himself decrees that even the animals should partake in this act of humility!

This spontaneous overhaul challenges a common presumption that humanity's heart is naturally resistant to changes in behavior or belief. The Ninevites, once obstinate in their wrongdoing, change in an instant, a testament to the often-untapped capacity for goodness when sincerity calls for it.

Plus, ancient Assyrians (like those in Nineveh) had reputations for severe cruelty, adding another layer of astonishment to their conversion—a reminder that change is possible regardless of one’s history. This story echoes down through history as a clarion call about redemption—illustrating that no one is beyond the reach of transformation when open to a heartfelt, genuine turn towards something better.