I've heard it said before that the root of all sin is pride.  Whether it's true or not, pride certainly has been the cause of downfall for many people and kingdoms.  The warning from Obadiah is that the pride of the heart has deceived Israel.  See, pride has a way of making us believe that we know what's best for ourselves and for our lives.  Pride causes us to think that it's all about us and that we're all that matters.  When a person believes this, things in his or her life begin to take a downward spiral.  Israel became proud as a nation, and as a result, she lost God's blessing.  There is no place for pride in the Kingdom of God because God shares His glory with no man.