The statement your mother made makes absolutely no sense.
Even if Atheism WAS a religion - it would still make absolutely no sense as you can still be part of a religion and NOT believe in a Deity.
She failed on both counts. But I'm sure you still love her :)
I suspect that she didn't didn't think it through. An oxymoron requires two mutually contradictory aspects of a whole concept. Atheism is a single aspect, the absence of a belief in a deity. There's nothing for it to be self-contradictory with. To do so, you'd have to add an additional aspect, such as an atheist who believes in a god.
Perhaps she's simply falling for the common misconception that atheists actually believe in a god, but are just angry at him/her.
I suspect the "contradiction" in her mind is between a. god exists and b. you think he doesn't. That would actually be an example of irony, but I don't think she understands that.
Religious people think that the existence of god is "obvious" and "undeniable" because there is "no other explanation" for the universe. They treat the two issues as one in the same and engage in very circular reasoning regarding them.
This is why oxymorons consist of two words, like "silent scream" or the humorous "military intelligence."
Or "Microsoft Works"
That's a good one!
Moria, if you really want to get your digs in, just say:
"Do you even know what an oxymoron is? It's a phrase containing two inherently contradictory concepts, like 'religious logic', not a word that stands for single concept, like 'atheism'."