I decided to both self-identify and "come out" as an atheist back in the autumn of 2009, though those close to me knew I was a practicing agnostic for many years before. I stated my reasons for it in a lengthy Facebook note, and even set my "religious views" (uh oh, it's official now!) on my profile to "atheist" instead of something clever like "none, thanks" or just leaving the thing blank because oversharing on Facebook is stupid anyway.
My reasoning at the time still stands today, though my views are ever-evolving.
Why "atheist" as opposed to "agnostic" about something inherently unknowable? It's not because I "know" there is no higher being, but because I find myself utterly unable to believe any account I've heard of a higher being's existence. To say I'm "agnostic" in regards to the various religions would be like saying I'm agnostic about werewolves or faeries or unicorns. If I can truthfully say I'm agnostic about any viewpoint of the sort, it's deism, because the idea of a non-interfering Creator behind the very laws of physics that guided the formation of the universe is so abstract that it's really not a belief at all as I see it.
Why "atheist" as opposed to "naturalist" or "materialist" or any other term denoting belief in only the visible as opposed to disbelief in the invisible? It's not because my worldview is defined solely by what I don't believe; I do, however, wear the label for the pragmatic reason that it sets me apart politically from the theists. As an American, I live in one of the most religious nations in the western world, whose motto reads "In God We Trust" and to whose flag we are told to pledge allegiance as "one nation under God." I'm personally opposed to the idea that believing in God is a prerequisite to being American, and I would feel like I was silently helping perpetuate that notion by not standing up and saying, "Atheists exist in America, and I'm one of them!"
Why "atheist" as opposed to "none" when prompted for my religious affiliation? Ditto the reasons in the last paragraph, plus the fact that I've decided I want the visibility. Many people are afraid to draw attention to themselves when they don't fall in line with the crowd, and I honestly can't blame them. It makes a person a target for all manner of flak, and while I've been lucky to have tolerant, understanding people in my life, other people in other situations have a strong incentive to not rock the boat. Just as secular humanist organizations are putting up billboards across the country saying "Don't believe in God? You're not alone," I want to try to send a simple, reassuring message to anyone in my social sphere who feels isolated by their disbelief.
The word "atheist" still has a negative connotation in popular culture, evoking images of bitter, angry, acerbic people with a vendetta against all things religious. And yes, I sometimes even feel that way, and sometimes lash out with unkind words against religion as a whole when I see people do amoral or unethical things in the name of their faith. The atheist community has no shortage of voices who make that sort of vitriol their calling card - but there are also many of us who want to have dialogue instead of debate with the religious majority that surrounds us.
I'm not so naïve that I think the word "atheist" will ever be completely free of any negative connotation. I simply want to help spread the idea that atheism is not a monolithic mindset, but rather a broad term encompassing a variety of philosophies connected only by the fact that they don't include a deity. Right now, in the minds of many, it is a label meant to divide "us" from "them" across a clear-cut boundary. My hope is that more people will realize that the statement "I don't believe in any god" tells you as much about a person as does the vague statement "I believe in some god or other." Honestly, I hate using labels to categorize human beings, and the more nebulous a label's connotations become, the more it forces us human beings to get to know one another before passing judgment.