The problems with defining religion play a central role in the argument that you’ve been developing over your last two books. Why can’t we—as Supreme Court Justice Potter Stewart famously said of pornography—simply know it when we see it?
The word “religion” comes out of a particular history. There are various ways of telling that history, but one could say, from the perspective of someone like me who is interested in church/state issues, that the notion that religion is a discrete, bounded aspect of human culture and society is something that emerged in the early modern period, parallel with the emergence of the modern state. With the secularization of the state and the differentiation of socio-cultural formations within society, religion gets reinvented as something separate. But the context in which that happens shapes what religion means. Politically, it comes to serve the modern state by providing a location in which modern citizens are trained to be moral, functioning members of society. This is a very particular understanding of religion, rooted in a particular kind of Protestant Christianity. Naturally, once modern societies try to expand that role beyond Protestant Christianity, they begin bumping up against different understandings of where religion ought to fit.