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.
Nevertheless, Jonah's deliverance was indeed a marvelous sign in its own right; and it is to the eternal credit of the Ninevites that they honored God by believing it.  The action of the Ninevites in this matter proved to be an accurate prophecy of what would happen in the days of the Lord Jesus Christ.  It was the deliverance of Jonah which convinced them; and it was the resurrection of Jesus Christ which convinced the whole Gentile world of the power and godhead of the Son of God.