Charles Ellicott Commentary Jonah 3:5

"And the people of Nineveh believed God; and they proclaimed a fast, and put on sackcloth, from the greatest of them even to the least of them." — Jonah 3:5 (ASV)

Believed God. —Or, believed in God. Notice again an implied contrast to the dullness of the Jews, who were “slow to believe” the prophetic warnings addressed to themselves.

Proclaimed a fast. —Apparently on a spontaneous resolution of the people themselves. (See Note to Jonah 3:6.) The fast would no doubt be for one day, according to the Jewish and the general Eastern custom.