This is from Feet of Clay by Anthony Storr, as quoted in Under the Banner of Heaven by Jon Krakauer:
Both revelation and delusion are attempts at the solution of problems. Artists and scientists realize that no solution is ever final, but that each new creative step points the way to the next artistic or scientific problem. In contrast, those who embrace religious revelations and delusional systems tend to see them as unshakeable and permanent. […] Religious faith is an answer to the problem of life […] The majority of mankind want or need some all-embracing belief system which purports to provide an answer to life’s mysteries, and are not necessarily dismayed by the discovery that their belief system, which they proclaim as “the truth,” is incompatible with the beliefs of other people. One man’s faith is another man’s delusion […] Whether a belief is considered to be a delusion or not depends partly upon the intensity with which it is defended, and partly upon the numbers of people subscribing to it.
Who is having those revelations?
Critical examination of the lives and beliefs of gurus demonstrates that our psychiatric labels and our conceptions of what is or is not mental illness are woefully inadequate. How for example does one distinguish an unorthodox or bizarre faith from delusion? Gurus are isolated people, dependent upon their disciples with no possibility of being disciplined by a church or criticised by contemporaries. They are above the law. The guru usurps the place of god. Whether gurus have suffered from manic depressive illness, schizophrenia or any other form of recognised diagnosable mental illness is interesting, but ultimately unimportant. What distinguishes gurus from more orthodox teachers is not their manic depressive mood swings, not their thought disorders, not their delusional beliefs, not their hallucinatory visions, not their mystical states of ecstasy. It is their narcissism.
Nehaveigur 