Few intelligent people differ on whether or not the ocean exists. Or even whether or not the atmosphere exists (even though we can’t see it).
Reciprocally, few intelligent people have differing opinions on the existence of unicorns.
God (as the term is generally used by intelligent people) isn’t akin to ocean or atmosphere (the folks who profess belief do not consider themselves to be describing something that can be apprehended by the physical senses). Theist and atheist alike tend to agree on that.
The atheist of simplistic depth-of-thought on the subject, especially if of strongly materialist bent, may consider that sufficient to conclude that therefore God does not exist.
The theist of more or less equally simplistic depth-of-thought may not be offering much clarification — after all, the atheist probably comprehends abstractions such as “efficiency”, “beauty”, or “justice” and does not reject them out-of-hand as nonexistent just because you can’t directly see, taste, smell, hear, or dropkick them.
Atheists of the sort you’re likely to find on this board are more often quick to grant that “God” is a word that might apply to something other than an entity that ought to be providing physical, verifiable manifestations [visibility, dropkickability, etc], and that “God” could apply to an abstraction, but they will generally reply that, a), theists generally and historically are not saying that the “God” of which they speak is an abstraction, and that they are being reasonable in insisting that the subject of the debate is “God” as the term is conventionally used and not “God” in some esoteric & atypical usage of the word as claimed by this or that SDMB theist; and b), that theists tend to claim communication of some sort takes place with this “God”, and abstractions don’t generally run around chatting up people, so we’re back to entities — people or critters — and they should be leaving footprints or critter droppings or be visible in some portion of the electromagnetic spectrum, etc etc, unless the theists can explain this away.
Theists of the nonshallow-thinking variety are most likely to say that throughout history “God” has in fact been used for a lot more than the baby-talk semi-translucent bearded fellow, and that even if (as a consequence of the institutionalization of religion and periods of obligatory lip-service to religious tenets) there have been lots of physical-literalist-entity types of beliefs about “God”, they have legitimate reasons for thinking their more abstract and complex experience that they call “God” is, in fact, that which was sensed (albeit not with the physical senses) and named by people throughout human history and given that name, “God”, among others. And such theists reject the binary categorization of all things into either an abstraction or something with which people can communicate — even if nothing other than God can be described as an abstraction to whom one prays and from whom one receives communication, that doesn’t prevent categorizing God thusly.
Once you’ve gone that far into the thicket, you’re not so much having an argument about whether or not “God exists” but whether or not to use the word “God” to reference such experiences, whether or not you yourself have had such an experience, and what sort of valuation to place on such experiences, your own and those of other people.
It has been said of the different religions that they are like climbing a mountain — i.e., that the religions are paths, not final destinations, that the correctness of one path doesn’t negate the correctness of another path, and that as you ascend you start to see how the paths converge, etc etc. … I would submit that atheism and theism are also two ways of climbing the mountain and that at a certain point you stop seeing the other as “wrong” and start to see both as converging attempts to understand and make sense of life and world.
You will notice quite a bit of that on this board — atheists and theists alike conveying their newfound respects for their opposite numbers on this board and how they’ve come to respect the other viewpoint at least in its better manifestations, and how they will no longer dismiss the entirety of the other perspective on the basis of louder but less reasonable proponents and their behaviors and arguments.