Just caught up to this and I felt like I needed to point this out.
"a set of beliefs concerning the cause, nature, and purpose of the universe,"
Atheism is not a set of beliefs, or a system of beliefs, or a doctrine or anything of the sort. It is a single belief (which itself is simply the rejection of a belief). Thus, this definition does not apply.
If every single belief is a religion, then the word 'religion' loses all meaning, as it becomes simply a synonym for 'belief'. My belief that Coca Cola is superior to Pepsi becomes a religion. My belief that olives are over-rated becomes a religion. My belief that Star Wars is the true word of God... well, okay, maybe that one IS a religion, but my point still stands.
Atheism makes no claims as to the cause, nature or purpose of the universe, it is simply a rejection of beliefs submitted.
I think the best way I've ever heard it put is "Atheism is a religion like abstinence is a sex position."
Lack of belief God is a belief in no God. Ephiral made this point a while ago.
No it is not. It may seem a minor semantics, but its two entirely different things. Given how many debates such as this I've seen boil down to semantics, its probably good to be clear on what is meant by words such as 'atheism'. When ones claims that 'Atheism is a religion', they're utilizing a definition of the word that I think most people using it to identify themselves don't mean.
"I do not believe there is a god" is not the same as "I believe there is no god."
One is the rejection of a claim, the other is a claim itself. The former requires no faith, the latter does (or some kind of crazy mounds of evidence that simply do not exist.)
Admittedly people who hold both of those positions will likely both call themselves 'atheist' as the word has a history of being very poorly defined. I do think that MOST atheists, at least my self most certainly, subscribe to the former position more than the latter. As I believe Ephiral as pointed out, an argument based on certainty is effective insanity. I suppose a less certain statement of "I believe there is no god", as in, "I think its likely there is no god" is more reasonable, assuming its meant as more of a strong statement of belief, "There is no god", that is simply an insane statement to make, in my opinion.
Please note, I am using the term 'god' to refer to any deity, not specifically the God we all know so well in the Western World. Admittedly I would say I believe that God does NOT exist, simply based on my observations of the world around me not really meshing with that particular deity's existence. In other words, I feel as though sufficient evidence exists to declare that assertion to be false. As for whether or not SOME form of deity exists... I simply believe that there is no evidence in favor of it, thus I reject the assertion until such a time that new evidence is made available to me.