...erroneous beliefs that usually involve a misinterpretation of perceptions or experiences. Their content may include a variety of themes (e.g., persecution, referential, somatic, religious, or grandiose)...The distinction between a delusion and a strongly held idea is sometimes difficult to make and depends in part on the degree of conviction with which the belief is held despite clear contradictory evidence regarding its veracity (p. 299).Kind of interesting that we aren't treating religious fundamentalists for their mental disorder, isn't it? Although some mental health professionals are willing to admit that there is considerable overlap between psychosis and intense religiosity, many believe that religion is a legitimate cultural issue that should not be viewed as pathological.
According to the DSM-IV-TR, "Clinicians...must take cultural differences into account. Ideas that may appear to be delusional in one culture (e.g., sorcery and witchcraft) may be commonly held in another" (p. 306). Thus, it appears that a belief which would otherwise be considered delusional should not be considered such if it is commonly held within a particular culture. Does this mean that Billy Graham is in good mental health while Tom Cruise is crazy?
Of course, I am deliberately oversimplifying the complexity of psychiatric diagnosis here. I do not believe that all religious believers are mentally ill or should be labeled as such. My point is simply this: as the intensity of fundamentalist religious beliefs increases, the boundary between religiosity and mental illness begins to blur.