Ya, that is a controversial title. This goes back to my days as a Religious Studies major, and the perennial problem of how to difine "religion" in general. This comes up now as Reconciler and Mitziut had a Theology on Tap together and are coming together to celebrate Shavuot/Pentecost.
As we sat around a bonfire and talked about God,I had the distinct experience that there was no thing called religion that we could say we held together. We were all perhaps religious (or in our current terms spiritual) but there wasn't either spirituality or religion in general, only the particular or positive instances of religion or spirituality.
This experience is potentially (as experience tends to be) mediated by my training in Religious Studies: At least in the early 1990's the discipline of Religious Studies questioned not the phenomenon of religious acts or religious, but found an overarching definition, or singular origin of these phenomena to be problematic and under question. It was an open question, but put from a phenomenological bias. That is if there was an underlying unity of the religious it needed to be shown through the phenomena, that is the particular manifestations of our subject of Study. Most of my professors and most of the text books we were assigned were concluding there was no such phenomenological unity of religious phenomena. The positive instances of religion could not be united except in abstract definition that failed to account for all positive instances of the phenomena. Accounting for Religion either was so broad as to simply include all forms of human activity or too narrow so as to exclude a positive instance.
A particular instance of this problem of definition of religion resides in one meaning of the latin religio: devotion to the gods. If one posits as a general definition and unity of all religion a belief in a deity or deities, one runs into problems, in at least two instances. The first particular instance is certain forms of Buddhism particular Chan or Zen Buddhism for whom the ultimate reality isn't a deity per se. One can perhaps stretch the definition of deity in such a way so as to include the Chan Buddhist understanding of ultimate reality, but that can strain the definition of god or deity. At the other end are certain forms of animism which treat all things as inspirited but don't even have an overarching ultimate unified reality or over-spirit. This is just one part of possible definition and understanding of a general unity of religion.
Yet, religio also had to do with a binding, of binding oneself or being bound to something. What I experienced around the campfire and talking about God was more along this line. For those of us there our understanding of God was bounded by our commitments, and we were committed to God in particular and differing ways.
This is then possibly also a meditation oriented around a Christian perspective. Christianity was first called the Way. A particular path to which one was bound, and from which one would not deviate.
My thought here is that even in coming together even in attempting to speak together about God, what was most evident to me was the difference in our paths, and the ways in which we were bound. There are and were differences that simply cannot be ignored, the differences aren't what is in the way of our unity, we obliterate the difference and we have nothing around which to unify, in our differences we can come together and together talk about God, but in that activity our differing ways of being "religious" or "spiritual" are heightened, and there is no common thing we share beyond the particular instantiations of our spirituality and religious beliefs and practices.
I am coming to think the the problem of the definition of religion, is that there is no religion as such that exist universally apart from its particular instances. There is then no basis for a unity of religions and/or spirituality. We are stuck with our differences.