One way a theist can guarantee to have me grinding my teeth is by telling me that atheists, by definition, can have no 'moral compass'. This statement, apart from being mind-numbingly offensive, it just so obviously wrong I don't know where to start.
This article looks at how (especially social) animals enforce a series of rules during play, to avoid injury while practicing necessary life-skills, and then how breaking those rules leads to social consequences. Although it concentrates on canids for examples, (it does reference research into other animals as well,) it isn't hard to extrapolate how similar ape behaviour develops into our 'morality'.
This quote especially jumped out at me:
"...the vast majority of human moral behavior takes place "below the radar" of consciousness, and that rational judgment and self-reflection actually play very small roles in social interactions."
Well worth the read, with a good selection of titles for follow-up reading if interested. The big problem, of course, is that the theist who blithely proclaim atheists can have no moral compass won't read it, but what's new there?
Jessica Pierce and Marc Bekoff, "Moral In Tooth And Claw", The Chronicle of Higher Education, 2009