"So tell me about myself. Am I a Communist because I must be if I don't believe in a Grand Supernatural Authority Figure, or am I an atheist because my prior committment to the International Socialist Revolutionary Struggle demands it?"Magic 8-ball says "What? Need more info." Are you a Communist because you don't believe in a Grand Supernatural Authority Figure? No. But unless you prove otherwise, you are a materialist-determinist. If you need a definition of this, please ask.
Since all you know about me is that I'm atheist, I assume you believe that that necessarily implies materialism, which in turn necessarily implies determinism. Which somehow necessarily implies Communism.
What is your definition of "materialism"? Is it the same as "naturalism"?