No religious beliefs define atheism—no religious dogma, no religious practice, no religious rituals, no metaphysical claims. That's why it isn't A RELIGION. Why this is so difficult for some people, I don't know.
Last night, I said to my son, who is a software engineer (logic, Antifascist), "So, Steve, do you think atheism is a religion?" Without hesitation, he answered, "Of course not." I mean, it's a no-brainer to most people.
Incidentally, I am not an atheist, nor a theist. I can say that because I know what those two words mean.