Amos came from humble shepherd roots when God thrust him into the role of prophet to the kingdom of Israel.  This was during the time Israel was enjoying a peak in their material and political success, and so hearing Amos denounce their prideful smugness was treasonous.  To them, the day of the Lord would be a day of total triumph over all their enemies… for don’t you know, we’re the chosen race, and God’s superior group here on earth!  But Amos was warning them that because of their unbelief and wickedness, the day of the Lord would be a day of judgement instead.  Anytime elitism rears its ugly head…God’s judgement will follow.