Jamieson, Fausset and Brown on Jonah 3:9

9. Who can tell--(Compare Joe 2:14). Their acting on a vague possibility of God's mercy, without any special ground of encouragement, is the more remarkable instance of faith, as they had to break through long-rooted prejudices in giving up idols to seek Jehovah at all. The only ground which their ready faith rested on, was the fact of God sending one to warn them, instead of destroying them at once; this suggested the thought of a possibility of pardon. Hence they are cited by Christ as about to condemn in the judgment those who, with much greater light and privileges, yet repent not (Mt 12:41).