Let me go out on a limb and suggest a working definition of religious rhetoric: “Religious rhetoric is persuasive language to or about the supernatural that takes as its base assumption the existence of the supernatural.” If it does not assume the supernatural but is nevertheless about religion, it is rhetoric about religion--not religious rhetoric. If we say “Brian is religious” we mean Brian has cultural practices that assume a supernatural power; if we say “this rhetoric is religious,” we mean something similar.
This definition needs more flesh than I have time to give it right now. First, I need to defend the way I define religion itself: belief in supernatural beings, forces, or powers. This definition, for example, would make it impossible to identify as a religion marxism or capitalism or baseball or a number of other human endeavors often called, sloppily I think, religions.
Second, I need to explain more of what I mean by "persuasion," since I believe the very architecture of a mosque, church, synagogue, temple, prayer room, or other religious building constitutes a rhetorical strategy designed to invoke a particular experience in those who enter and worship in them.
In other words, by defining American religious rhetoric, I have opened, rather than closed, an important semantic debate. I hope others will participate with me in defining our terms. (A wiki might be better, eh?)