Every once in a while, one of my fellow atheists gets bent out of shape about starting God with a capital G. He -- the atheist not God -- demands to know why I follow this theistic trend. Well, aside from the fact that "God" is the character's name (I don't spell "Alice" as "alice"),
I capitalize God because ...
1. Communication works best when we follow rules. For instance, the symbol cat is just an arbitrary set of marking on a page or screen. There's no rational reason why that symbol must point to the image of a feline. I could arbitrarily decide that "cat" means "a three-legged stool." But since, conventionally, cat is a label for an animal, it makes sense for me to follow that convention, assuming my goal is getting you to understand what I'm writing about.
Though you'd probably understand me okay if I broke the conventional "God" rule and wrote "god" instead, my general practice is to follow tradition when it comes to symbols. Communication is strongly aided when we use words in traditional ways. Each break with tradition risks damaging the transfer of knowledge from one person to another.
2. I'm generally writing about some subject -- not about me. Let's say I'm writing about the middle ages, and I include this sentence: "Back then, people thought god granted devine rights to rulers." That quirky spelling of "God" calls attention to itself. It will likely cause some readers to think, "Ah, I get it. The author is an atheist, and so he's choosing to not capitalize 'God.'" Which means those readers are thinking about ME instead of about the middle ages. That's absolutely NOT what I want.
For these reasons, I also follow the standard practice of using "He" as God's pronoun, not "he."
Whether I like religion or not, I have grown up in a world in which it's influenced our culture and language in many, many ways. It would be a huge pain to bypass or thwart all those influences, and I doubt I could do it even if I wanted to. For instance, I have no problem talking about the year 100BC, even though, as an atheist, I don't believe Christ was devine, and I'm not even convinced that the historical Jesus existed. It's just become a traditional, convenient way to mark years and pinpoint a date.
I aslo don't want to constantly have to switch rules. I would have to do this if I chose to start "God" with a lower case letter but then quoted something where He was upper-cased. For instance, I would have to write, "Why does US currency say, 'In God we trust on it,' when not everyone in the US believes in god?"
Finally, not all atheists are political. Some are, of course, but I'm not. I have no interest in pushing my atheism on people or converting others to atheism. To me, writing "god" would feel like saying, "SEE! I'm PURPOSEFULLY bucking the trend to make a POINT!" But that bores me. Unlike some of my atheistic brethren, I'm totally fine with me being an atheist while others are theists.