Theists reject specific beliefs of other theists all the time, but this doesn't make them atheists, right? You do believe that, for instance, a Muslim is still not an atheist even if he rejects the existence of Odin and Thor (... and in fact every god but one), don't you? Theists believe in lots of things besides gods, so what you've given is less than useful without more details. What sorts of things believed in by theists do you think need to be rejected to make a person an atheist? I mean, if theists all happened to believe that it's cold outside, a person wouldn't become an atheist for thinking "no, it's warm", right?