I more or less responded "what are you talking about?"

Apparently, his impression of atheism is that isn't just about not believing in god, but actively informing people that gods don't exist and providing evidence. People who do not believe in god, but don't actively promote atheism, are not really atheists! Basically, he thinks atheists are missionaries.
I told him that this was nonsense, that atheism literally means to not believe in god and that anyone who doesn't believe in god (as opposed to "not being sure" or "undecided") is really an atheist, and that most atheists are not going to go out of their way to promote atheism, but are instead simply fighting against being forced to have their lives dictated by religion.
It's worth mentioning this person is quite irreligious - he might have a bit of lapsed Catholic about him, in the sense he's got some family members who probably expect him to show up at the occasional mass -- but other than that, he seems to have no time for religion, and even found my theory that most people don't really believe in God very amusing.
Anybody else have any experience with people expressing strange perceptions of atheism?