God shows His grace when individuals and nations repent and turn to Him.
3 The word of the Lord came to Jonah a second time: 2 "Get up! Go to the great city of Nineveh and preach the message that I tell you." 3 Jonah got up and went to Nineveh according to the Lord's command.
Now Nineveh was an extremely great city, a three-day walk. 4 Jonah set out on the first day of his walk in the city and proclaimed, "In forty days Nineveh will be demolished!"Jonah 3:1-4 CSB
All People Have an Urgent Need
Perhaps Jonah thought—as we sometimes think today—that because of his rebellion the Lord wouldn't want to use him again in future service. It was one thing for the Lord to graciously spare his life; it might be another for the Lord to put confidence in him again as a prophet. But that's not the way it was going to be.
The Ninevites' repentance demonstrated that Jonah's message had all the necessary elements to bring about spiritual change. Jesus's first sermon in Mark 1:15 features similar brevity: "The time is fulfilled, and the kingdom of God is at hand; repent and believe in the gospel." All that is required for belief is for someone to turn and trust.
Knowing that our time is short and life is brief, we need to share the gospel with care and urgency.
5 Then the people of Nineveh believed God. They proclaimed a fast and dressed in sackcloth—from the greatest of them to the least.
6 When word reached the king of Nineveh, he got up from his throne, took off his royal robe, covered himself with sackcloth, and sat in ashes. 7 Then he issued a decree in Nineveh:
By order of the king and his nobles: No person or animal, herd or flock, is to taste anything at all. They must not eat or drink water. 8 Furthermore, both people and animals must be covered with sackcloth, and everyone must call out earnestly to God. Each must turn from his evil ways and from his wrongdoing. 9 Who knows? God may turn and relent; he may turn from his burning anger so that we will not perish.
10 God saw their actions—that they had turned from their evil ways—so God relented from the disaster he had threatened them with. And he did not do it.Jonah 3:5-10 CSB
God Acts with Sovereign Mercy
Our responsibility is to witness. The results are up to God.
In the Old Testament era, fasting and wearing sackcloth were signs of seeking God and repentance. For example, Daniel sought the Lord "with fasting and sackcloth and ashes" (9:3). Joel called people to lament and cry out to God while fasting and wearing sackcloth (Joel 1:13-14).
God knew the Ninevites' belief in Him was being demonstrated in actions—that they had turned from their evil ways. The people had genuinely repented. So God mercifully relented from the disaster He had threatened to bring on them.