all words need "boundaries" at some point, especially if we're supposedly talking about a "thing". The vast amount of people throughout history have described a god as a "being/thing" with specific "properties". The vast amount of people in our current society believe in god as a "being/thing" as well. So when someone asks, "do you believe in god?", that's the definition most people assume.
Of course, if you feel the need to make up your own personal definition of "god", that's fine. Just know that it does kinda confuse things and make the word more useless, rather than clarifying anything. If we're debating the existence of Tolkien's hobbits, but you're defining hobbits as "the love that is in our hearts", then there's not really much to discuss.
of course, if we're just talking about god as a "concept", then you'll get no argument from me. Of course the concept exists. But I think it's fair to say that when someone asks "why do you believe in god?", they're generally referring to the existence of an actual being, not just a "concept" in our heads.
Which is perfectly fine and all if we're speaking metaphorically and/or referring to concepts in our head, and no nonbeliever will disagree with that.
But if we're discussing universal essence/spirit as a "thing" that exists entirely separate from the thoughts in our heads, then that's an entirely different issue and gets far more into "magic fantasy superstition" territory. But it's difficult to ascertain what someone is referring to, since the word "god" is sometimes used in nebulous, vague terms.
I suppose this is why I've always thought that "religious" people are really just folks who take metaphors and parables waaaay too literally