That depends very much upon the individual. Some people cleave to their atheism so strongly that I think it's fair to say it's religious fervor, or something very near it. Someone like Richard Dawkins isn't just a guy who doesn't happen to believe in God, he is dead certain that there is no God and makes it his business to belittle everyone who doesn't share his belief. Some people, though, just don't happen to believe in God and they don't really care if someone else does. I'm fine with saying Dawkins' version of atheism is basically a religion for him, not so much with the second category.