'God' is hard to define, because to say "God is..." or "God isn't..." is to instigate a lie. 'God' neither is nor isn't--those terms apply to how we think about, and more importantly talk about, for commuication purposes, things. The world. 'God' does not conform to our words, or our standards of verbal interaction. That's what it means for God to be ineffable.
We experience the world through a medium that we call the mind. For every thing that we experience, acknowledge, and encounter through that medium, we assign a symbol (an idea and a word) to express what it means, both to us as an individual and as a community, a culture, and a people. Occasionally we find things to which words cannot be ascribed.
"All the world is one bright pearl."* You, as an ascribing mind, are on the surface of the pearl, since you have ascribed yourself. Everything you ascribe, everything you imagine to be, and everything imagined to be not yet ascribed, is on the pearl. The entire world is the surface of the pearl, yet we imagine that much more lies beneath the surface, something that sustains the surface, because a surface must be sustained. A mind must have a support. But if a support can be imagined, it is only the surface of the pearl. It is an epistemological model.
The surface of the pearl is everything: the universe. Everything that is; everything that we know and imagine; everything that could be, should be, and would be; everything that was and will be. Everything devised by our minds.
*
http://www.thezensite.com/ZenTeachings/Dogen_Teachings/Shobogenzo/004ikkam.pdf
Without substance, the surface of the pearl collapses, yet here it is. Yet, again, when you find that there is nothing sustaining the surface, and that what is left, as shiny as it is, remains, you can begin to surrender the need for such a word and idea as 'God'.
Monotheism means 'only god,' it is the idea that something beneath the surface sustains and informs all that is (on the) surface.
Polytheism I am not so familiar with, but I imagine it to be constituant on the surface.
Pantheism is the most appealing. It is the shiny surface of the pearl.
Just one atheists' opinion, drunk with delusion.