Even though the Holy Spirit is the coauthor, still we know that the Bible is written for humans and in a human language. It therefore resents of the inherent limitations of the human mind and human language. The book of Jonah is a point in case.
One of the main messages of the prophet Jonah is the radical conversion of the people of Nineveh, starting from the king and his family. down to the last citizen. It was a case of a mass conversion.

Did they convert because they were scared of the destruction that Jonah was declaring, or because of the miracle of Jonah? Actually, we cannot tell. The Gospel makes us believe that Jonah was a “sign” to the Ninevites, because of the three days and three nights he had spent in the great fish’s belly. They converted fast to his word. And it was a real conversion, meaning a complete turn around: the Gospel talks about “conversion” and the book itself describes it as belief in God, proclamation of a fast and changing of their dress into that of a sackcloth.

I think this statement covers the most comprehensive range of change of the Ninevites’ heart. Belief in God implied the renunciation of their many idols, of their wrong priorities, their allegiances and obedience. Belief in God for the Ninevites implied giving their life a new meaning and goal.

Proclaiming a fast implied acknowledgement of their sinfulness and need of forgiveness, renunciation of their sinful past and world outlook.

Changing their dress in sackcloth shows their determination to change their lifestyle and choices. Such a conversion starts from the innermost of oneself and must manifest itself also in the exterior through the way they confronted situations, solved problems and tried to bring about a new social and spiritual environment. The book of Jonah describes this change as follows: “God saw their works and that they had turned from their evil way.” Is this not the fast defined by Isaiah in chapter 58? A comparison of this few statements with Is 58 would be very revealing and instructive.

In return, God shows his very nature by repenting “of the evil he had spoken and intended to do to them and he did it not.” (2:10)
A generous and merciful God that is never outdone by humans’ goodwill. He prompts sinners to repentance and waits patiently until they open themselves to forgiveness. That is why it is meaningless to speak about a change of mind in God: it is not God who needs to change his mind, but it is the humans that may be not ready to accept God’s mercy and compassion.

To a case of national disaster the Ninevites came up with a national response. It is not only natural disasters like the environmental degradation like the irreversibility of the desertification of the earth that should worry us but global change of attitudes towards God’s existence, sanctity of life, basic equality of humans, etc. These are situations that claim universal attention. And this three days are intended to call our attention to these grave situation and take definite steps of conversion: renewed belief in a loving and merciful God, our need to turn away from sin and a consistent world vision and lifestyle.