We believe there is only one religion of God that has appeared at various times and under the guise of various cultures of the past...but spiritually they are one.
"Spiritually they are one" is also the main position of philosophical Hinduism, but with rather different thinking where, for example, Jesus comes out as one of a progression or group of remarkable individuals (this cuts right across the teaching of standard Christianity, of course)
I long tried to reconcile myself to this idea as it is liberal, tolerant, and makes significant inroads into issues arising from living in a multi-faith society.
Personally, I can't do it, as I find not enough of a common thread, common values, agreement about the nature and demands of the deity, the nature and mode of salvation...
It is indeed possibly to produce a "harmony" of most (but not all) religions, but I observe that this is not a happy fusion but Procrustean surgery, bits of various faith having to be hacked off or twisted about to produce the "fit" The procedure assumes that nothing vital from any religion has been excised and discarded, and that no essential change in voice and message has occurred to to any of the source religions.
This is just not the case, and it can usually quickly be seen with such attempts which religion has been taken as the "most correct" one, to which the others have "been found" (less a limb or two) to agree.
It is a pity, but I can't with honesty, make the idea work.
Either one version of theistic religion is a far better match for reality than , the others, or none of them is a useful representation. There is no need to consider which, at this point: the sheer difference between them is sufficient.
Those taking a "mix" or a "cocktail" for their beliefs do not seem (to me) to be engaging with with the concept of the existence of God implying a particular nature for that deity. Of necessity? I think so.
(but that does not matter, immediately, if "religion" is being used as a social and psychological entity, on the human level. The connect with reality and a god existing or not does not quickly arise from this perspective.)
Again, the Hindu thought and approach to "oneness" is rather different. Non-duality, all appearances actually being only one thing is a core and fundamental doctrine. Therefore *whatever* apparent separation or difference, everything is one.
That's a whole different idea, even though it has powers of assimilation approaching those of the Borg.
Chris