A quibble: Buddhism is both a philosophy and a religion; It’s possible to accept the philosophy but not the religion, and be both Buddhist and atheist at the same time. I have an acquaintance who spent a few years in a Buddhist monastery who would scold me if I failed to mention this. 
Grouping atheists together is problematic. It’s like a Golf Club for people who don’t play golf. Most labels like “Communist” describe qualities or beliefs a person has, while A-Theism is a quality or belief a person does not have.
Science is a method with material application. We might describe religion as a method with material application too. Both are concepts, not real in the material sense, but have real power to influence people. I’d like to say there is a categorical difference between the concepts, but it’s a little hard to pin down. Science has defined rules bounding the concept, while religion has rules that are arbitrary in some sense (not arbitrary to the practitioner! 
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