It's shortly after dawn on February 15th on the remote island of Tanna in the South Pacific. The oppressive humidity and heat of the rainy season is already building, the pigs and chickens are slowly stirring, and - as on every February 15th for the last 45 years - one of the world's strangest religious ceremonies is about to take place.

The village of Sulphur Bay is waking up, as it does every morning, directly under an active volcano. The cone of Mount Yasur steeples up above it, thumping periodically as blisters of magma burst inside its crater, and scattering ash onto the dead plains around its base like a carbon snowfall. On the coastal side of the village is a black sand beach running with steaming rivulets of scalding spring water, too hot to touch but ideal for washing clothes and dishes. Between the devil and the deep, the palm and thatch huts are arranged quite untypically for a Melanesian village: not around a central clan hut or banyan tree but framing a large, deserted square like a parade ground. This is because Sulphur Bay is one of a handful of villages in this part of the world where the people neither worship the Christ of the missionaries nor practice the traditional kastom (custom) religion of their ancestors, but who live with a god of their own: a spirit messiah known as John Frum.

John Frum is the son of God, but he's not Jesus. He's a black Melanesian, but sometimes a white man - or, according to others, a black American GI. He's a kastom messiah, come to turn the people of Tanna back to their old ways before the missionaries - but he's also a universal avatar of change, a successor to Buddha or Jesus or Mohammed. Like Jesus, he's poised to return - or, perhaps, he's already here. He's a volcano god, with an army of the dead who live down in the crater, and a spirit who approaches the men of Tanna when they drink their intoxicating kava and bring their spirits into communion with him. Back in the days of colonial rule when he first appeared, the British thought he was one of the locals dressing up and spouting nonsense to foment rebellion. They arrested a succession of 'troublemakers', pillorying them before their community to expose the deception, but the locals knew perfectly well that John Frum was neither this man nor that one. Apart from anything else, he continued to appear. So, a new tactic: anyone who was found to be talking John Frum nonsense was hauled off to jail in Port Vila, the administrative capital over a hundred miles away. But these 'ringleaders' became martyrs to the growing religion, and the stories of how John appeared to them in jail are now part of the canon of oral traditions, hymns and revelations of the new religion.

To anthropologists, John Frum was an example of one of the strangest and most exotic phenomena to be observed in traditional cultures: the cargo cult. All across Melanesia, from New Guinea to the Solomon Islands to Tanna's archipelago, the New Hebrides, dozens of unconnected communities, thousands of miles apart and speaking unrelated languages, seemed spontaneously to generate the same set of bizarre beliefs. A new dispensation was on the way, when the white man would vanish from the islands, and his cargo - Western goods - would be diverted by magical means to the local people, who were its rightful owners.

'Cargo cults' got into full swing during the 1950s, though once the phenomenon had been classified by Westerners it seemed that the beginnings of the movement could be traced way back, as far as the 1890s. The classic account was by the Australian anthropologist Peter Lawrence who went out to the Madang district of New Guinea in 1949 to conduct field research into the traditional social relations of people who, despite colonial rule, were still living much as they had in the recent Stone Age. Lawrence gradually discovered that his presence in Madang had become woven into an extraordinary complex of beliefs. Persistent rumours abounded that a cargo ship was about to arrive in the harbour with huge consignments of goods for him, and the local people asked him to help them supervise the clearing of an airstrip. When he asked what the airstrip was for, he was told that cargo planes were about to arrive bringing tinned meat, rice, tools, tobacco and a machine for making electric light. And when he asked who was sending this cargo, they replied 'God in Heaven'.

Lawrence's analysis of what was going on came to constitute a template for what seemed to Westerners an inexplicable and repetitive complex of delusions. First came 'cargo belief': the apocalyptic conviction that the world was about to turn upside down, the islanders finally receiving the material rewards of the white planters and administrators who were currently enjoying the fruits of the black man's labours. Then, even more puzzling, came 'cargo ritual': new religious practices designed to reel in the cargo by magical repetition of the acts which were currently bringing it to the white man. All over, islanders were downing tools, clearing airstrips in the jungle, building imitation radio masts out of bamboo, scouring their bibles for hidden messages, even sitting around politely drinking afternoon tea. If it worked for the white man, so the theory went, it would work for them.

All across the region, colonial governments cracked down hard, rounding up the cargo prophets and imprisoning them. Planters and other Western commercial interests, who coined the term 'cargo cult', saw it all as madness which demonstrated the ignorance and superstition of their workforce. More liberal whites attempted to explain to the locals that cargo wasn't produced by magic, but by hard work and the product of generations of technological progress, and the only way in which Melanesian societies were going to become rich in cargo was by working and earning it. To the locals, the subtext of this explanation was clear: the whites were politely refusing to give up the secret of their cargo.

On Tanna, this secret was vouchsafed by John Frum.

The various stories of John's first appearance tend to contradict each other: all are oral traditions, amplified by Chinese whispers, reinterpreted by different prophets and constantly trimmed to fit the nature of the religion as it developed. The colonial version is that the 'madness' began in 1940, when certain locals began painting their faces white, wearing hats and appearing to their fellows at night, prophesying in falsetto voices about the coming expulsion of the whites and arrival of huge quantities of cargo. Most Tannese versions explain that John Frum had been witnessed for many years before this, but they had kept him secret from the authorities, as his message was that the people needed to turn back to their kastom beliefs which the missionaries had prohibited for a century.

But by 1941 there was no doubt that something was going on. A 'prophet' named Manehevi from Sulphur Bay had been arrested and tied to a tree for a day by the colonial administration, pour encourager les autres, but John Frum continued to appear. Subsequent witnesses had been deported and imprisoned. Then, big news: a huge detachment of American troops had arrived on the neighbouring island of Santo. Not only had these Americans brought unheard-of amounts of cargo - arms, tanks, boats, food, medicine - but a considerable number of them were black. The centuries of unbroken symmetry between foreigners (white, rich) and locals (black, poor) had been broken, and the black GIs were variously interpreted as descendants of the islanders who'd been kidnapped by plantation owners in the past, or as John Frum's own detachment of the US army. Messianic fervour gripped Sulphur Bay, and one Sunday morning the new movement came out into the open with a baffling act of civil disobedience which sent shock-waves through the white community. The compulsory attendance at the Presbyterian church was universally ignored; instead, a group of locals walked solemnly into the white-owned trading post and carefully removed every price label from the stock.

The effect of this surreal outrage was to shift colonial tactics from bullying to a belated attempt at education. American officers were drafted in to explain that John Frum was nothing to do with the American army, and that if they wanted cargo they were going to have to work for it. The locals were underwhelmed, and the build-up to Judgement Day accelerated. In a repeat of a ritual which spontaneously generated itself dozens of times across the region, the John Frum 'cultists' began to destroy all their Western possessions, throwing their hard-earned money into the sea. This common motif of 'cargo ritual' could perhaps be seen as an act of faith and preparation for the new dispensation (John Frum would bring new money, with a coconut stamped on it), or as an adaptation of the traditional potlatch system of Melanesian culture where such sacrifice puts your fellow-man, and by extension the world, into your debt. But however oblique its motive, its effect as a gesture of resistance was total. If the natives are destroying their money, even the most stubborn colonist can hardly ignore the message that they've lost control of their workforce.

The crisis gradually devolved into an inconclusive stand-off: John Frum didn't arrive with the promised cargo, but neither did the Tannese return to the churches or mission schools. In Sulphur Bay and the surrounding villages, the Sabbath migrated to Friday, on which day John Frum was celebrated with singing, dancing and kava-drinking, a joyful inversion of the dour Presbyterian past. The movement gradually manifested itself with the restitution of various kastom practices, which the administration had no choice but to tolerate. Finally, on February 15th 1957, an American flag was ceremonially raised in Sulphur Bay and the new religion officially announced to the outside world.

It's this flag-raising which still forms the ceremonial centrepiece to John Frum Day. Shortly after dawn, the day is blessed with a service: a hundred people crowd into a small hut on the edge of the parade ground and sing some of John Frum's hymns which have been 'channelled' over the years, some of them miraculously in tongues unknown to the receiver. These have a sombre, lilting quality reminiscent of their Presbyterian roots, but the ceremony is plainly not Christian: the participants, fingers raised, each place a flower on the shrine at the climax of the service. While the hymns are sung in near-darkness, the rest of the village goes about its normal morning routines: pigs root around, women wash clothes in the hot springs, men gather in small groups under the trees, the village store opens to display its stock of tinned pilchards, Omo and second-hand nylon shorts.

Slowly, a hum of activity begins to form around the 'Headquarters', a corrugated iron shack on the opposite side of the parade ground from the service hut. John Frum elders emerge dressed in old US Army uniforms, some sporting medals and an array of sewn patches, everything from Greenpeace logos to black American baseball stars. A couple of them are pointed out to me as sons and grandsons of the original John Frum prophets. The Headquarters is the command centre not just for Sulphur Bay but for the entire religion, the modest Vatican of the John Frum Movement. It's a strictly supervised male-only domain, in line with both the kastom tradition of the male clan-hut and the initiatic all-male secret societies which take various forms across the islands. Consequently it's a mystery what actually goes on in there - though an English traveller I spoke to claimed to have had a glimpse inside and seen the elders clustered round a video and TV, running off the village solar panel and playing Knights of the Round Table with French subtitles.

A crowd gathers, the chiefs gradually assemble into a parade formation, and two flags are brought out of the Headquarters. One is an old wartime US stars and stripes, the other the flag of occupied Vanuatu, the resistance movement which led the nation to its independence in 1980. Three of the elders process slowly to the flagpole, and the flags are raised and saluted in a faithful facsimile of US military ceremony. The rest of the elders stand stiffly on parade, awaiting the next stage of the ritual: the John Frum Army.

From the jungle above the village, the army appears: 40 or so of the younger men, in jeans and bare torsos with 'USA' painted on their chests and backs in day-glo pink magic marker. Each carries a bamboo 'rifle', a long stick with its tip sharpened and painted red, with which they present arms before the elders. They're marched out onto the parade ground, where a second set of flags is raised: the US Marines' insignia, and the state flag of Georgia. The elders put the army through their paces with shouts and whistles, marching them back and forth under the now-baking sun and the volcano's plume of steam, each barked command to turn and shoulder arms applauded wildly by the crowd. Part military drill, part kastom dance, it's a performance like no other, genuinely stirring for islanders and visitors alike.

Eventually the Army march back into the jungle, but the elders remain on the parade ground and summon the rest of the village to perform more kastom dances. Troupes of a dozen or so young men and women take turn to process and wheel in circles, marshalled by the whistles of the chiefs and accompanied by strummed guitars. The Army return in gaggles, chatting and grinning proudly, supplementing the USA slogans on their chests with fern head-dresses and tinsel streamers and joining in the free-form dancing. The day gradually devolves into the traditional kastom revels of dancing, singing and eating glutinous lap-lap porridge.

Among the spectators is Vanuatu's Minister of Health, who's the local member of parliament and a passionate and articulate defender of the John Frum movement. He could have been at a summit meeting in Japan today, discussing the proposed (and American-sponsored) blacklisting of Pacific tax havens like Vanuatu, but on February 15th there's nowhere more important for him to be than Sulphur Bay. "The John Frum Movement were the first heroes of the independence movement," he explains. "They were the first people to speak out against colonial rule. John Frum's message was that we should return to kastom ways, to keep our own identity."

This is a view which outside observers began to develop during the 1960s, as interest in traditional cultures and the anti-colonial struggle grew. David Attenborough, visiting Sulphur Bay in 1960 for his Quest in Paradise TV series, could dismiss John Frum as a "sinister cult", "pathetically childish", and no more than an "act of defiance towards the mission", but the next generation of anthropologists began to interpret it as a response to a broader struggle. The history of white contact had been, for the Tannese, unremittingly grim: murderous explorers and adventurers followed by slavers and sandalwood traders, followed in turn by "blackbirders" who kidnapped the islanders for indentured labour in the Queensland cane-fields, and Presbyterian missionaries who forbade dancing, kava, traditional dress and ritual, enforcing their prohibitions with a rod of iron which gradually developed into colonial administration. Throughout this history the islanders, disparate peoples speaking unrelated tongues, had no language or structure for forming a group identity and no shared belief system which could operate as the basis for a coalition. Even their religions had no name until the pidgin term kastom was coined to describe them. 'Cargo cults', across the South Pacific, began to be reinterpreted in the West as the evolutionary first stage of labour unions, separatist movements, lobbies for democratic and political reform.

But this view of John Frum as an avatar of political independence is shared by few islanders outside the Movement. Everyone I spoke to in the rest of Vanuatu seemed to share the view which the Minister imputed to the old colonial dispensation: that John Frum is something of an embarrassment to the progressive-minded and politically engaged Vanuatu of today. "This is from the age of our grandfathers," one told me, "before we understood how politics works, and how to get independence for ourselves." The David Attenborough view of the superstitious native waiting hopelessly for his cargo remains the perception of most of the other islanders. "John Frum was just a black American GI", another told me. "But you can't tell the John Frum people that. You can't tell them he wasn't a god."

But the charge that John Frum is a cargo cult is one to which the Minister of Health has a robust response. He insists not just that John Frum is a cargo cult no longer, but that it never was one in the first place. "We Tannese knew about God before the missionaries came. Of course we knew that there was a creator, who made all this. But the whites could never understand that there could be a revelation for the black man, a messiah for the man of Tanna. They could only imagine it had to be about their consumer goods. Cargo is not our obsession, it's the white man's." And the American flags, the parade we've just witnessed? "This is not about waiting for cargo to come from America. This is a reminder of our history. The Americans helped us, they freed our leaders from the colonial jails." It's true that the US Army treated the islanders far better than the colonial rulers, paying far above the standard wage and dispensing free Red Cross aid, but the nature of the alliance between the John Frum Movement and the Marines is less than clear. This being the 21st century, the Minister of Health intends to set the record straight in the near future on the Vanuatu government website.

Nor are the younger John Frum members backwards or insular in their view of their religion. "God appeared to the people of Tanna in the form of John Frum", one tells me, "Just as he appeared to the Jews as Jesus, and to the Indians as Buddha. It's the same message, the same revelation. Our colonial struggle has happened all across the world, across all different cultures. John Frum is the messiah of all of us who have had the same struggle."

But this ecumenical take on the Movement highlights what is perhaps its central mystery: why here? All over the world, kastom and Christianity have come into conflict. In hundreds of cultures, Christianity has meshed with the old religion, producing versions sometimes so idiosyncratic as to be almost unrecognisable. In dozens of cultures, too, the yoke of Christianity has been thrown off and the kastom ways returned to. But where else apart from Tanna has the collision produced a third entity, an entirely new religion, with its own indigenous kastom messiah?

The strains of these unions and contradictions have frequently told throughout the history of the John Frum movement, and its various prophets have rarely sung from the same hymn sheet. John has been black and white, man and spirit, Tannese and American. Cargo has been imminent for some, a metaphor for others. But this year has seen a schism on a grander scale than any before, one which has split the village of Sulphur Bay in two. One of its chiefs, Isacwon, has decamped half a mile up the hill into the forest, taking several dozen families with him. Their village, now six months old, seems well-established, and its first John Frum Day finds it festooned with flowers and decorations. But it has no central parade ground, and no marching John Frum army: its celebration is a return to kastom in the time before the missionaries.