I noticed some years ago that many statements about God made perfect sense to me if I substituted "Mother Nature" for "God", and that's when I realized that I wasn't an atheist after all, just a believer in a natural as opposed to a super-natural god. That leaves us with a god that is just as awesomely powerful, just as unknowable (or very nearly), and just as omnipresent, as is the Christian or Jewish God.
The god I believe in does however differ from the Christian God that I learned about in Sunday-school in a number of ways. The first difference is that my god is feminine, the Christian God masculine. There is no holy-book to tell us what my god approves of and what she disapproves of. So how can we know? Do we look around to see what works and what doesn't work, and assume that what succeeds is what Mother Nature approves of?
Some questions have no simple answer. When asked if I believe in God with Yes and No being the only choices, then "No" may well be the less misleading? Which of the choices is less misleading depends on what people mean by "God": in our Christain (or Judeo-Christian) society almost everyone "knows" that god is supernatural, and to believe in a natural god is not to believe in God. I recall getting a lift from a Baptist minister/missionary (I'd run out of petrol) who asked if I believed in God; I replied "not in a supernatural god". My answer demonstrates that, although neither Yes nor No is a satisfactory answer, only a few words are needed to give a nearly adequate answer. Incidentally, the title of this essay is misleading; with the Judeo-Christian notion of God it's less misleading for me to call myself an atheist than not.
Am I also a Humanist? Nope. Humanists vary in their regard for nature but for the most part they are just as wrong-headed as Christians, in thinking that one species (Homo sapiens) is all-important.
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Note: when to write "God" (capitalized) and when "god" (uncapitalized) is not easily decided.
Similar titles: Martin Gardner wrote an essay "Why I am Not an Atheist", after Bertrand Russell's book "Why I am Not a Christian".