In particular, Cavanaugh contends that the belief that there is something inherent in religion that makes it more prone to violence than the "secular" is false. Secular governments (e.g., the USSR, the People's Republic of China, the Khmer Rouge) have killed just as many people (probably more) than have religious ones. To be clear, Cavanaugh doesn't argue that religious ideologies have not been or cannot be violent. He clearly recognizes that they can. He simply agues that the notion that there is something about religious ideologies that make them more prone to violence than secular ideologies is just plain wrong. However, because many believe that religion has a propensity to promote violence, there is a widely-held assumption that it has to be tamed by restricting it access to public power.
Cavanaugh also argues that the myth of religious violence is just that: a narrative that is taken for granted and seldom challenged. He contends that the separation of the "religious" from the "secular" focuses attention on certain forms of violence (i.e., religious violence) while directing attention away from others (i.e., secular). The consequences of this is the religious violence is often seen as fanatical, while secular violence is seen as rational, necessary, and sometimes laudable. Cavanaugh's point is not that one is necessarily better than the other. Both can be unjust. The problem is that the "myth of religious violence" is so prevalent is that many of us accept the use of secular violence without ever challenging its legitimacy. With this modest book, he's hoping to change that.