They are killing non-believers. That makes it religious.
Only the islamist would act as the final word of God AGAINST Gods creations.
They are fighting the modern world in the name of Islam. But muslims also kill each other in order to win some kind of edge. We are not fighting them in the name of any God. People would like to believe that we are in a religious war, because that is a simple concept. We are in a War for the future of the World not any religion, on our side. And we are not fighting against any nation state. Terrorists are in a category of their own.