Atheism is not disbelief in any particular religion, or even religion in general. Atheism is the disbelief in a supernatural deity. It is against theism. Most atheists are also opposed to religion, per se, but there can be religious atheists and non-religious theists.

To be agnostic you essentially place equal weight to the arguments for god as for the arguments against. Essentially agnostics just can't make up their mind. There are agnostics who are truly ambivalent, but there are also many who are atheist but don't know it or want to admit it.