If you were an atheist, you wouldn’t consider atheism a religion, just like not playing basketball isn’t considered a sport.
While that may be true for the 'live and let live' brand of atheists, it is not true for the more liberal brand which mixes their atheism with their politics. Their dogmas, sacraments and articles of faith are more clearly defined than your average ultra Christian fundamentalist or even Taliban Muslim. And more extreme and intolerant.
Look at what happened to the former CEO of Mozilla.