I think, if it's necessary to put any sort of "label" on it, it's probably best described as a philosophy. That said, however, I doubt really that it even satisfies the criteria for a philosophy either, never mind a religion.
Atheism is an absence of belief in a supernatural being. That's all. If some people feel it necessary to proselytize this then in my own opinion they're simply naive and misguided. This is the sort of ideology that people have to reach on their own if they ever do; you're not going to "enlighten" somebody who either has been indoctrinated into a religion from birth or certainly someone who has chosen a religion later in life. To try to alter that through some sort of reasoning (read more likely browbeating) process simply won't work. Moreover, nobody has the right to do that, from either side.
I absolutely hate the term, "tolerance", too. We should respect others' beliefs, not merely tolerate them. That doesn't mean that we have to adopt them as our own, but we must respect their right to those beliefs, regardless of whether or not we share them.
