OK, then the terms for us are synonymous, which is part of my argument. They only ceased to be synonyms from the 17th century on, which is event if one examines the texts. Denys Turner on the Unknowingness of God presents that argument quite skilfully.Not bad, Thomas.The only difference is that what you are calling religion, I choose to call spiritual.
The problem is twofold:
The first is that those who declare themselves 'spiritual but not religious' invariably and (in my experience) unfailingly, regard 'religion' and those who profess a religion only in its most negative sense — that is with an undisguised prejudice that sees nothing good, beneficial nor positive, and which in any other context would be absolutely unacceptable — take religion away and replace it with race, or colour...
In fact, the assumption, often stated quite explicitly, that those who follow a religion do not and cannot think for themselves (that 'blind faith' is another term for mental deficiency or intellectual surrender) and so on, is offensive for the same reason — and yet I meet that attitude here often, and often by those who believe themselves to be quite 'right on'!
There is another aspect in that many who declare themselves to be 'spiritual but not religious' do this to put themselves at distance from those who declare themselves religious (they'd be horrified if anyone made that mistake — prejudice again) but at the same time assert that no-one has the right to question or comment on their particular brand or expression of spirituality.
The loss of this sense of the supernatural, the loss of the language of symbol, and the failure to see certain symbols as the embodied transcendent, as spiritual forms if such a thing was possible, allows the void to be filled by relative and contingent shades which can be hugely gratifying, but which are not always what they seem to be.
Spirituality of the transcendent order, that is spirituality as understood in the context of religion, is formless and inaccessible to the senses, so anyone making such a declaration is talking of a different order and a lesser order of thing altogether.
Again, we're back with a culture that has been indoctrinated into materialist consumerism, and sees experience as a necessary benchmark of the real, which is, of course, far from the case.
The question then is if the order of transcendent spirituality of which the Religions speaks has no sensible component, nor any intelligible content, what exactly does one mean by being 'spiritual'?
I think the kind of spirituality people mean is a 'sense of' or 'spirit of occasion'; the feeling at watching a sunrise, of looking at the Milky Way, a baby reaching out to grab a finger. I don't mean just sentimentalism (that is often a very important part of it), I mean invariably they take that 'feeling' to be the primary constituent of the spiritual process, and seek for that as a marker of authentic spirituality, as if bliss were a signifier, whereas in fact what has happened is the authentic spiritual signal now has been reduced to sentimentalism.
I would argue that if we're talking an order of engagement with the Divine such as has been spoken by St John (e.g. "and it hath not yet appeared what we shall be" 1 John 3:2), by St Paul, and by the greater mystics of the Christian Tradition (St Denys, Meister Eckhart ...) then that is not at all the same as people saying 'I am spiritual but not religious' because in the traditional Christian sense no-one would or could make that self-designation.
Which is where authentic 'comparative religion' come into its own. Everything I've argued (apart from the chronology), is axiomatic in Hindu metaphysics, if anything moreso, which is why Guénon declared that Hindu metaphysics is a universal metaphysic ...So Forms and Archetypes and Yes/No form the very essence of Western Thought.
Please be aware that I am the one refuting a distinction into this/that with regard to the question, I'm saying religion and spirituality is and/and, and if one extricates one aspect only, either way, one has neither.
God bless
Thomas