What is the point of an atheist spelling “God” in lowercase? To show that he doesn’t believe in him? But in common punctuational parlance, spelling something in lowercase doesn’t mean that you don’t believe in it. It just means that it’s a common noun rather than a proper noun. So “god”, in accord with punctuational convention, means you’re referring to any one of a number of gods rather than any particular one; whereas “God” signifies reference to the Judeo-Christian God in particular.
I don’t refuse to capitalize things I don’t believe in or like. I capitalize “Narnia” though I know it’s fictional. Likewise “Harry Potter”. I don’t refuse to capitalize “Adolph Hitler”, though I despise his memory. Nor do I spell “Allah” in lowercase, nor “Muhammad”, nor “Martin Luther” for that matter. Heck, I even capitalize “Satan”. : )