The standard replies:
"If atheism is a religion, then not collecting stamps is a hobby."
"If atheism is a religion, then baldness is a hair color."
Those replies are good, but I like the following one better. I'm not sure where I came across it, but I think it was from something Austin Cline wrote. I can't find the exact source.
"If you think atheism is a religion, do you think 'theism' is a religion too?"
What are the religious doctrines of "theism"? It doesn't make sense to call theism or atheism religions. It's like saying "communism" is a country.