This is the original quote- a well-known atheist apothegm.
"I contend that we are all atheists. I just believe in one fewer god than you. When you understand why you dismiss all the other possible gods, you will understand why I dismiss yours."
The real irksome thing to most atheists, certainly to myself personally, is that in the general case, an atheist knows a great deal about religions and their methods and their histories and their fables and even their rituals and 'secrets'- because, one of the interesting side-effects of atheism is the search for meaning, which is only possible when the cultural icon of a god is removed from the explanation.
So, saying that an atheist does not know religion is usually, _usually_, insulting. And I, for one, take it that way.