So the people of Nineveh believed God, and proclaimed a fast, and put on sackcloth, from the greatest of them even to the least of them. (v. 5)

Jonah was sent with a particular message to a particular people. One must be careful to always speak that which the Lord gives them, nothing less, and nothing more. Remember, it was the Word that Jonah preached that brought repentance, and not the man himself, or his delivery. We put too much stock in methods!
The Lord cautioned him to preach the “preaching that I bid thee” (v.2) and Jonah was careful to say exactly that. We do not know all that God told him, but he did tell them that they would be destroyed shortly, in forty days. It was a message of doom. The Bible says that the people of Nineveh “believed God” and repented. Folks, men always were saved by faith. Their faith led them to repentance. Look at how they prayed “mightily unto God” and turned from their wicked ways, (2 Chron. 7:14), and found mercy in the eyes of the Lord: But let man and beast be covered with sackcloth, and cry mightily unto God: yea, let them turn every one from his evil way, and from the violence that is in their hands. (v. 8) These folks were serious in their repentance! Not only did they pray, but they also fasted. Now I am not sure how potent a forced fast is with the Lord, but He seems to have honored the faith of the people each time a public fast was proclaimed.

True fasting seems to contain a certain element of sorrow. Always when we see a people fasting in the Bible, it is not for wisdom, or financial needs, but for sorrow of heart. It is the sorrowful heart that compels a compassionate God to move on our behalf. Nineveh was definitely sorrowful to think that they would all be destroyed in forty days, but they were also afraid. Great fear compels great fasting, and prayer unto our Great God. Any time we read about “sackcloth” the theme is great sorrow. In foreign countries when the king was sorrowful, the entire country was to be sorrowful; when he fasted, the people fasted, whether they understood the prOBlem or not. Though I do no like to compare the preacher to the king, when he proclaims a fast, we all need to participate. He is our leader in spiritual things.