Thanks for the thoughtful reply, @Windwalker .
While I find a certain elegance in the elephant metaphor, I don't agree with it. Why assume there is only one elephant in the first place? That's a very monotheist, or monist, assumption.
Do you believe that Hindus are monotheists or polytheists? The parable of the Blind Men and the Elephant, comes out of Hinduism, I believe.
Why not assume two? Or three? Or four? There isn't any particular reason to assume one over the other, when you get right down to it.
This is a very good question. But first, I think the challenge of that question is in the terms of theism itself, and the very word "God" itself. When someone says monotheism, as opposed to polytheism, it suggests to the mind we are speaking of deity forms, entities, or beings. The gods, in other words. Are there many such gods, or deities, or only one such God, a single being, or a single entity?
Does monotheism in its deepest understanding, separate the Divine from us as an entity? Is "God" in the monotheistic sense a being, a deity? Does God has a wrapper around it that defines it outside of creation and all its creatures, be those gross level or subtle level creatures? To most Christians I speak with, it does. They see God as external to themselves, in the way they might think of any of the gods as beings outside themselves.
I would think that a deeper understanding of monotheism would be in the ways the Hindus understand that all the gods, are expressions of that one single Divine Reality, or Brahman. It's the same with the Tao. It's the same with the Buddhists Emptiness, or Shunyata. And it can be the same in the Christian sense of "godhead", or Divine essence.
So then understanding that "God" is pointing to the Source of all that is, the Emptiness, or Void, or the Abyss, that all forms arise out from, that single Source embraces all that is, and is not separate from anything, including all the gods. Everything, including all the gods, humans, animals, plants, minerals, atoms, quarks, and strings, arises and have definition, form, and being, only in relation to the same Source.
That to me is monotheism. Not one god, versus many gods. Those are still gods, or deity forms apart from other forms. It is the difference between forms and formlessness itself.
The best analogy I can think of is the empty paper on which words are written. Without that fabric of reality itself, nothing can have existence. For anything to have existence, it has to be drawn upon "something", which is this case is not a thing at all, but no-thing. That is the same "emptiness", or blank paper, no matter what shape of form or meaning or being, or anything whatsoever we are talking about, that is part of every aspect of their existence. The words have the paper in every line and curve of their letters. Without it, they are not anything at all.
That is what I see as "God". Like the Hindus, we can have many gods, but only one Source, or Brahman, one Divine Reality. But even using the word "one" is wholly incorrect. It is not one as opposed to many. But it is both the One and the Many, and neither one, nor many. It is not an it. nor is it the all. It is, "Is'ness". And that is the same no matter what it is we are looking at, or experiencing. Hence "one" conveys "sameness". Unity within all diversity.
So finally, why one Source versus many sources? This is still tricky to explain. I believe in what the gods represent as aspects of reality, particularly as archetypal forms of what is in all of us. I would think of them as subtle-level ancestors in that sense. But with any of our own ancestors, they all share a common source at some point in deep history and time. They all share commonalities, such as existence and being itself.
The monotheists can tell the story how they want, yeah, but when there's insistence on that being the only way to tell the story - that there's just one elephant and if you disagree you're just wrong or mistaken - is annoying.
They are still thinking of the monotheist God as a being or an entity. Hence, only our entity is true entity. That is historical how monotheism began, with YHWH being the principle deity among the others, in a henotheistic way. Eventually, the others were not real entities, and there was only one entity, and so forth.
But the mystical realizations of that deity, saw behind and beyond that mask, or that form God takes to our minds, and God became truly universal, in the sense of Source, Ground of Being, Shunyata, Nirguna Brahman, and so forth.
But these are mystical realizations. Beliefs and ideas about God, do not capture that realization, as language and concepts separates God apart from ourselves and other things as a thing itself. But as the Hindus say, "Neti Neti", not this, not that, to remind us that it cannot be understood apart from anything else as a thing itself. It is not "a God". It is not "a" anything.
I can play those head games too and allege the worshipers of the one-god are "actually" worshiping many gods they mistake as one. I don't do that, because unlike the monotheists, polytheists don't have a habit of proselytizing.
In a sense you are right. They are seeing God as a god, just the right god. That's not any different than tribal gods at war with each other. "Our god is the best god". But rather than using clubs and spears, they use apologetics.