HOW do people tell the difference between the genuine Messiah and a madman? Picking through the wreckage of Waco, this appears to be easy. David Koresh was certainly mad, wasn't he? Cases of mass suicide, such as at Jonestown in 1978, which left 900 dead, and the estimate that more than 1,000 cults have started since the Second World War, suggest that many people have a problem distinguishing between the authentic and the false mystic.

Matthew's exhortation to beware false prophets (Matthew vii,15) indicates that this is not a new problem.

Psychiatrists know well that religious experience is a symptom of severe mental illness, including schizophrenia and manic depression. Even R D Laing, a psychiatrist with a cult following in the Sixties, declared that the distinction between mysticism and madness was not easy.

William James, an eminent 19th- century US psychologist, noted it was impossible to find historic figures from established religions with no record of psychiatric symptoms, including Martin Luther, George Fox (the founder of Quakerism), and John Wesley.

The mystical experiences found in St Augustine's Confessions (AD354 to 430) have been compared to modern accounts of psychotic breakdown, as have experiences of the Old Testament Hebrew prophets: Moses heard a voice from a burning bush telling him he was to be a servant of the Lord (Exodus iii,2-iv,17); in India, in the sixth century BC, Siddhartha became Buddha after mystical experiences under the bo tree.

Medieval mystics such as St Francis of Assisi and St Catherine of Siena have been diagnosed as hysterics by contemporary clinicians. Stigmata - sometimes open wounds paralleling the wounds on the body of Christ at the crucifixion - have been seen on those in psychiatric wards.

Another possibility is that divine figures of history, although not mad most of the time, had psychotic episodes during which they had vivid but sustaining experiences. A troubling implication of Jesus's 40 days and nights in the desert is that he suffered psychiatric problems, such as conversations with the devil. This has been discussed regularly in medical circles, including in a serious paper called 'The Mental Health of Jesus Christ', in The Journal of Psychology and Theology, in the Seventies.

The rise of modern secular medicine coincides with the decline in prophets. This raises the question of whether contemporary seers are in psychiatric wards around the country. Two thousand years ago Jesus received a crown of thorns. Today the Messianic have electro-convulsive therapy. Psychedelic drugs can also generate mystical experiences.

Research psychiatrists have found that heightened perception, sudden revelations as to 'real meaning in the world', feelings of communion with the divine and exultation are common to mystics and psychotics.

The difference appears to lie in the incoherency of thought that characterises the insane, and their tendency to hear voices. The legitimate mystical state can be typified by ideas with some logical connection and by hallucinations. The psychotic's God tends to be wrathful; a punitive force, rather than the benevolent or loving God of the mystic. But a would-be follower can hardly undertake such examinations to evaluate the local messiah. A simpler test would be to try to check on mystics' claims of supernatural powers - most commonly prophecy and healing. But there is still scope for opinion and coincidence.

Nostradamus, a 16th-century French physician and prophet, is widely considered the most accurate soothsayer, but experts differ in their interpretations of the prophecies. These are written in numbered quatrains. Quatrain 9 was thought by one expert to refer to the role of Haile Selassie in the Second World War, while others were convinced the same prophecy was about Henry IV.

Some predictions are likely to come true by sheer chance; believers tend to focus on the successful ones while ignoring the others. A study of predictions made by 92 psychics, astrologers and other seers about earthquakes in the US were compared with predictions randomly generated by a computer. The seers' predictions were no more accurate than the computer's.

The best-known Christian site of spiritual healing is Lourdes in France, where, ever since a peasant girl saw the Virgin Mary in 1858, throngs of pilgrims have visited. Catholic commissions on healings at Lourdes have pronounced 11 cases 'miraculous'. But when pilgrims' medical records were investigated in 1957 by Donald West, a psychic investigator, he concluded: 'There are no cases of lost eyes or amputated legs sprouting anew . . .'

In all cases of religious healing subjected to formal medical examination, changes can be easily explained by natural remission or psychological placebo effects. There are instances where people wrote testimonials of their cure that were then used by their healers for years after their death from the disease of which they had been 'cured'.

This kind of healer is neither a mystic nor genuinely insane. They are characterised by a desire to exploit the credulity of others for personal gain. David Koresh and Jim Jones resemble this group. A true mystic is never self-seeking and leads a life of poverty and hardship dedicated to helping others.

The ambition to be the next Messiah seems an increasingly competitive and attractive career choice. One patient who had recovered from the delusion of being God said in the British Journal of Psychiatry in 1991: 'I liked to imagine it because I felt so useless without it . . . I still feel inadequate now - I had always felt everything I said was worthless, but as Jesus everything I said was important.'

The writer is a psychiatrist at the Institute of Psychiatry, London.

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