There's a difference between "I don't believe there is a god" and "I believe there is no god". It's subtle, and difficult to explain, but it's there.
That said, I never said it's a religion, but it is a belief.
you are correct, there is a difference between lack of belief of X and believing the contrary of X. mainly, the difference is that "believing the contrary of X" requires "lack of belief of X". In other words, lack of belief is not suffcient for believing the contrary but it is necessary.
therefore, there is a factor or set of factors that must occur for "I don't believe there is a god" to become "I believe there is no god".
Even so, strictly speaking, you are wrong about atheism being a belief. The set of factors I mentioned do not apply here. The strong definition of atheism only implies lack of belief. If you want to know more, look up "shifting the burden of proof" as relates to religious arguments.
Atheism is an opposing belief... Agnostic is the absence of belief (or rather, the belief it's not accessible to humans, so that you have atheist agnostics and theists agnostics, but well...).
(not sure where I stand between those twos, I mostly don't care, so I haven't think a lot about it...)
As hurricanes said on the last page, this is not correct. You're misusing the terms. Agnosticism is not lack of belief, but rather lack of knowledge. In the same vein, atheism is simply lack of belief. There are many different ways people adopt atheistic ideas, and I'm sure many people DO hold the belief that "There are no gods". But again, strictly speaking, that belief is outside the meaning of the word "atheism".