Atheist supremacy is an anti-atheism troll phrase. It smuggles in the claim that atheists (or at least those who openly declare themselves as such) believe they are generally superior to non-atheists (i.e. religious people, agnostics, and passive atheists with religious sympathies) and therefore entitled to rule or control them -- a tactic designed to demonize atheists and inspire fear of them.
There do not appear to be any substantial arguments behind this phrase; it is clearly guilt by association (atheists are supremacists) and claim-smuggling (the connection is never stated outright, much less justified).
Although I was not able to find any actual arguments explaining this phrase (which is of course one of the things that makes it so specially trollish), the lyrics of a Gilbert and Sullivan parody entitled "I Am The Very Model Of An Atheist Supremacist" convey the general idea. --Woozle 16:07, 15 March 2009 (UTC)