Atheism isn't just a position on a single issue.
When you are an Atheist, there is a set of morals and beliefs that come with it, just like any other religion. This set may be far broader and varied than Christianity, but it is still a limited set. There is a distinct set of assumptions you can make about the beliefs and ideals of a person who is Atheist that you cannot make about an arbitrary person in general. That makes it enough of a religion using my definition or yours.
Unless you believe that you can be an Atheist that has the exact same set of morals and ideals as a Christian, where the only difference is that you don't believe in a God or afterlife or any of that.
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_foolmo_
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