I really don't believe that religious belief has that much to do with it, its more like the attraction of power through gang-culture. Normally internal squabbles and power struggles break these thing apart before they get too powerful but the ruthlessness and a lot of luck seems to have helped ISIS.
That DD, must be the general opinion . For those who have no roots in religion it must be very hard to appreciate, but these people will die for such inherent beliefs.
To listen to people talk about 'sky fairies' and joke about there not being Jesus or a God, even for someone like myself who is not fanatically religious and always open to the questions it brings, such remarks actually bring tears to my eyes through maybe desperation, frustration or even anger , I'm not sure, but the same remarks will never convince me otherwise and I believe followers of the twisted Islam who are fanatical will not relent. It's totally impossible.
I suppose it could be explained as you telling someone your mother never gave birth to you. Try and convince everyone else. It's that difficult.