Idiosyncratic belief systems which are shared by only a few adherents are likely to be regarded as delusional. Belief systems which may be just as irrational but which are shared by millions are called world religions. When comparing the beliefs held by psychotics with the religious beliefs held by normal people, it is impossible to say that one set of beliefs is delusional while the other is sane.
Anthony Storr (1920-2001) English psychiatrist and author
Feet of Clay: Saints, Sinners and Madmen, ch. 10 (1996)
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