Atheism isn't broad enough to be a religion unless "atheism" is defined artificially to be equated with secular humanism or a civil religion such as Americanism. Atheism is just skepticism towards the ideas of all theistic religions. But that's not to say that most atheists don't have functional, atheistic religions or irrational faiths.
I'm a compatibilist, so I'd explain freewill as a type of self-control that's compatible with determinism. Determinism, too, has to be redefined, because "natural law" is deistic, causal explanations are more like pragmatic models, David Hume showed we have no empirical basis for saying there are necessary connections in nature, and quantum indeterminacy undermines the Newtonian clockwork view of causality.
Through the link below I talk about the related view of eliminativism, which says all of folk psychology (consciousness, freewill, meaning, purpose) is illusory.