Who knows whether God will not turn and repent, and turn away from his fierce anger, so that we might not perish?"
Many have marveled at the fact that the repentance of the Ninevites did not last; but, as far as we know, it lasted for that generation, and the blessing of God was continued for an extended time afterward.  In fact, the greatest period of Nineveh's power and prosperity came a century after the events of this chapter.  The eventual falling of the whole nation into debauchery and violence again was nothing more than the normal human reaction to God's blessings, nations finding it quite easy to renounce God and all righteousness in times of prosperity, and thus making the very blessing of the Father the occasion of their turning away from him.  It may also be surmised that Israel, herself increasingly hardened and sinful, offered no encouragement to Gentile converts.