Were Jehovah to fail to fulfill his promises, the Devil would have an excuse to call God a liar who withholds good and thus rules in an unfair way.
فَٱلشَّيْطَانُ ٱدَّعَى أَنَّ ٱللهَ كَاذِبٌ وَظَالِمٌ وَيَمْنَعُ ٱلْخَيْرَ عَنِ ٱلْبَشَرِ .‏