It appears from this, and other passages of Scripture, that the most express declarations of God’s displeasure against sinners, still afford ground and room for repentance. Thus in the prophecy of Ezekiel 33:14-15, “Again, when I say unto the wicked, Thou shalt surely die; if he turn from his sin, and do that which is lawful and right…he shall surely live, he shall not die”; and again, in the prophecy of Jeremiah 18:7-8, “At what instant I shall speak concerning a nation, and concerning a kingdom, to pluck up, and to pull down, and to destroy it; if that nation, against whom I have pronounced, turn from their evil, I will repent of the evil that I thought to do unto them.”

The Lord God speaks to us by His Word in plain and popular language. He condescends to our feeble apprehensions. God cannot repent; He is of one mind. Who can turn Him? Yet when afflictive providences lead men to a sense of their sins and to an acknowledgement of their demerits, and excite a spirit of humiliation, repentance, and prayer, He often mercifully changes His dispensations and averts from them the impending evil. Such was the effect of Jonah’s message to the Ninevites. The people humbled themselves and repented of their wickedness; and God suspended the execution of the sentence which He had pronounced against them (3:10).

My brethren, may we not fear that the men of Nineveh will rise up in judgment against us and condemn us, if we do not imitate their example and humble ourselves before God? They repented at the preaching of Jonah immediately, on their first hearing him; and they sought for mercy upon a peradventure, when they could say no more than, “Who can tell, whether there may be the least room to hope for it, after what the prophet so solemnly declared?”

John Newton:
The Imminent Danger and Only Sure Resource of Our Nation
from: http://www.chapellibrary.org/

             How great is the power of God over the hearts of men! Nineveh was the capital of a powerful empire. The inhabitants were heathens. The many prophets who, during a long series of years, had spoken in the name of the Lord to His professed people of Judah and Israel, had spoken almost in vain. The messengers were often mocked, and their message despised. The inhabitants of Nineveh, it is probable, had never seen a true prophet till Jonah was sent to them. If they had reasoned on his prediction, they might have thought it very improbable that a great city, the head of a great kingdom and in a time of peace, could be in danger of an overthrow within forty days. But it is said, they “believed God” (Jon 3:5). The awful denunciation made a general, universal impression. The king arose from his throne, laid aside his robes, covered himself with sackcloth, and sat in ashes (3:6). A sudden cessation of business and pleasure took place; he proclaimed a strict fast, the rigor of which was extended even to the cattle. His subjects readily complied and unanimously concurred in crying for mercy, though they had no encouragement but a peradventure, “Who can tell if God will turn and repent, and turn away from his fierce anger, that we perish not?” (3:9).

John Newton:
The Imminent Danger and Only Sure Resource of Our Nation

from:  http://www.chapellibrary.org/