Pastor Ritesh Joshua had just called a tea break when he saw the men in the saffron scarves. More than a hundred, some wielding sticks, had massed outside his white stucco church on the outskirts of Gorakhpur, a temple town in eastern Uttar Pradesh. It was three days after Christmas.

“They started shouting, ‘You are converting people. We will not allow any conversions here’,” he says. “They shoved people, turned over furniture, and told me, ‘You are the main culprit’.”

The men, allegedly part of a religious activist group called the Hindu Yuva Vahini, cornered one of the parishioners. Smartphone footage shows the woman pulling her blue shawl tightly around herself as she answers questions about her involvement with the church. “No one is forcing me to convert,” she insists.

“If the police hadn’t arrived, we don’t know what would have happened next,” Joshua says. “After the men left, everyone in the church was silent, so frightened. This is a time of testing for us.”

Last week, the monk who founded the HYV, and whose firebrand Hindu supremacist vision guides the organisation, was selected by the party of prime minister Narendra Modi to lead the most populous state in India – the equivalent of the sixth largest nation on earth.

Yogi Adityanath’s appointment as chief minister of Uttar Pradesh, about a fifth of whose 200 million people are Muslim, is “stunning”, says Milan Vaishnav, at the Carnegie Endowment for International Peace, a foreign policy thinktank. “He is an extremist in terms of his speeches, a very proud rabble-rouser, and somebody who doesn’t have a claim to fame other than a dedication to a strident form of Hindu nationalism.”

“It is an important and disturbing moment,” agrees Ramachandra Guha, an author and historian. “It is the fringe moving to the mainstream.”

The boyish face of Adityanath, 44, beamed down on Gorakhpur last week from thousands of green-and-saffron banners plastered along its main road. On Sunday, tens of thousands of people are expected to line the road for his triumphant return to Gorakhpur, the electorate he has represented for almost two decades in the Uttar Pradesh parliament.

Another addition to the city streets last week were squads of police officers hunting so-called “Romeos”. Along with a ban on buffalo slaughter, cracking down on amorous young men was a key campaign promise of the ruling Bharatiya Janata party. Officially, the police are targeting “Eve-teasing”, the endemic sexual harassment that blights some Indian streets. But critics instead see a crackdown on mixed-religion couples, in line with Adityanath’s fevered, baseless warnings that Muslim men are trying to seduce Hindu women as part of “love jihad”.

Facebook Twitter Pinterest Yogi Adityanath, the hardline Hindu leader, prepares to meet party leaders. Photograph: Manish Swarup/AP

The surprise appointment of Adityanath to run the state has deeply rattled Manoj Singh, a Gorakhpur journalist who has spent the last two decades tracking the new chief minister and the HYV men he labels a “private army”. He recalls, 10 years ago, when the city boiled with religious tension after the murder of a Hindu man, and Adityanath rose to address a crowd of HYV supporters outside the Gorakhpur railway station.

“We cannot tolerate such incidents any more,” he told the men. “It has crossed all limits. If someone sets ablaze the houses and shops of Hindus, then I do not think that someone stops you from doing such things.

“Get ready for a final battle,” he says. Court documents allege Adityanath’s followers then went on a rampage, burning Muslim-owned properties and an Islamic mausoleum. “I saw the burned shops,” Singh says. “I saw the Muslim men who ran the shops trying to douse the fire. I knew one of the shopkeepers. He was very emotional. He said, ‘Look what has happened to me. I’m ruined’.”

Adityanath was arrested and imprisoned for 11 days. He broke down in parliament recalling the ordeal. But, Singh says, his fiery rhetoric was unchanged. “If [Muslims] kill one Hindu man, then we will kill 100 Muslim men,” he has said since. But Adityanath began to distance himself from frontline violence. “He took a political turn,” Singh says. “He started having political dreams.”

Hinduism is a poor soil for fundamentalists such as Adityanath to grow. The world’s third most-practised religion has no pope, no mandatory scripture, no impulse to convert new believers. The caste system has sown division deep into its DNA. Wherever Hinduism has taken and flourished across Asia it has blended with and infused local cultures, forming what author Sunil Khilnani has called a “bewildering internal pluralism”.

It was contact with more rigid doctrines, first the Islam of the Mughals, then the Christianity of the British, that first planted the seeds of political Hinduism. They grew with demands for Indian independence, as those who sought freedom for the extraordinarily diverse subcontinent grappled with the question: what was an Indian, anyway?

Jawarharlal Nehru, India’s first prime minister, opted for the broadest possible answer. The India his Congress party advocated was, he wrote, proudly plural: “An ancient palimpsest on which layer upon layer of thought and reverie had been inscribed, and yet no succeeding layer had completely hidden or erased what had been written previously”.

But Hindu nationalists such as Vinayak Savarkar, discerned in the countless communities that populated modern-day India, Pakistan and Bangladesh an essential “Hindutva”, or Hindu-ness, that persisted no matter what faith an Indian practised.

“Religious minorities will all have the right to practise their religion”, Savarkar wrote of the India he envisioned – but they were inescapably citizens of a Hindu “rashtra”, or nation.

“This vision of Indian history is one of victimhood,” says Guha. “That Hindus were first persecuted by the Muslims, then the British, and they can only recover when they repudiate all that is Muslim and British in their past.”

The elevation of Adityanath is part of that “old battle between the Congress and the rightwing Hindu parties”, he says. For the first 40 years after independence, Hindu nationalists struggled to summon more than 10% of the national vote. But their appeal has surged in the past quarter-century, culminating in the election three years ago of Modi, the staunchest Hindutva flag bearer ever to occupy the prime minister’s residence.

Other than Modi’s political talents, Guha says the growth of Hindu nationalism is partly down to poor leadership in the Congress party, whose most prominent leader is Nehru’s great-grandson, Rahul Gandhi.

“But it is also part of a regional and global phenomenon of religious nationalism. You see it now in Turkey, and in our neighbourhood, with Pakistan and Bangladesh. There are parallels with Sri Lanka. And even in America, when George Bush said Jesus was his favourite philosopher,You can’t blame Rahul Gandhi for everything,” he says.

Modi was briefly an international pariah over his Gujarat state government’s alleged role in ignoring, and possibly abetting, deadly Hindu-Muslim riots in the state in 2002. But he assiduously reinvented his image in the decade before winning power in 2014, projecting himself as a pro-business, Apple Watch-sporting statesman obsessed with economic development.

In power, Modi has been coy about his Hindu nationalist agenda, prioritising issues such as tax reform and corruption crackdowns over the national ban on cow slaughter his party championed on the campaign trail.

With the selection of Adityanath, “the veil has been lifted”, says Vaishnav, from the Carnegie Endowment. “It answers one of the questions that we had about Modi all along,” he says. “Is this guy’s project about development or Hindu nationalism? What this pick reaffirms is that it’s not an either/or question. He has two faces: one is Modi the great economic moderniser, and the other is one of muscular nationalism – and Adit is its starkest manifestation,” he says.

Facebook Twitter Pinterest Yogi Adityanath, left, with the party president Amit Shah in Delhi. Photograph: Rajat Gupta/EPA

In Zafara Bazar, a Muslim district of Gorakhpur, Gulshan Ali is talking bitterly near the butcher shop where he worked until last Monday: “They talked about ‘development for all’, but the moment Adityanath became chief minister he started taking away our jobs,” he says. That was when – less than 24 hours after Adityanath was sworn in – police officers told him the business was being shut. “We didn’t get any notice,” another butcher, Jawad Ali, says. He pleaded that his shop sold only buffalo, not the cow meat that many Hindus eschew. “But they told me, ‘From today, your business is closed’.”

A thick blanket now hangs over Jawad Ali’s shopfront, and he passes his days with other out-of-work butchers reading the newspaper and gossiping darkly about what might be coming next. “For several generations we’ve been butchers,” he says. He admits he has been operating his shop unlicensed for the 15 years – but not for lack of trying. “Since 2002 the government stopped renewing meat licences because of Yogi Adityanath and his movement,” he says.

A previous government, one that relied on Muslim votes to hold office, worked out a compromise between its voter base and the growing clamour to ban cow and buffalo meat in the state: butchers such as Ali would be denied licences, but allowed to continue running their businesses.

The bargain held until Adityanath’s unexpected ascension. The crackdown on butchers has left up to 2,500 families in Gorakhpur without an income.

Heightening their frustration is that India is the world’s largest exporter of buffalo meat, with most of the companies run by Hindus who see no clash with their beliefs. “Here they’ve found a new god in buffalo,” one of the meat-workers mutters.

The chief preoccupation for many Muslims in the city is what comes next for the HYV. A few kilometres from Zafar Bazar is the resplendent Gorakhnath Mutt, a campus of ornate, chalky white temples interspersed with manmade ponds and patches of yellow and saffron marigolds.

The temple, which Adityanath oversees as chief priest, was buzzing this week with political officials and HYV men basking in the glow of their leader’s sudden promotion. “You talk to many Muslims, in and around the campus here, they all appreciate that Yogi Adityanath has become chief minister,” says Pramod Kumar Mall, the officer in charge of the HYV.

The role of the HYV, now that its leader is the most powerful man in Uttar Pradesh, will not change, says HYV officer Pramod Kumar Mall. “We are working for the nationalist movement. We don’t want this country to disintegrate. There are so many movements who want to disintegrate the system, and we want to stop them and make people understand about it,” he says.

He will definitely favour Hindus over Muslims ... this is what he has been since the beginning Wijahat Kareem

Regrettably, he says, there are “many” Muslims in the country working against Indian interests. “Just as President Trump has found so many, in India you will find so many.” But he is adamant that minorities in the state have nothing to fear from Adityanath’s rule. “This country belongs to them,” he says. “[As long as] they feel they are citizens of this country and feel they should respect the national religion – just as Hinduism has accepted many religions.”

Despite Mall’s assurances, Muslim community leaders in Gorakhpur are well aware of the new reality in their state. Over tea at his home, surgeon Wijahat Kareem, 62, describes his own political philosophy as “Gandhian”. “But Gandhi is losing his sheen,” he says. He chooses his words carefully. “You cannot change his heart,” he says of the new chief minister. “He will definitely favour Hindus over Muslims, but we can’t complain. This is what he has been since the beginning. You know with whom you are talking But there is hope that because of his past record he will be more cautious, more liberal than he was earlier on,” he says.

Hope, he concedes, is all Uttar Pradesh’s Muslims have left to rely on. “Politicians cannot win on the basis of Muslim votes,” he says. “So we have to keep believing in the right-thinking Hindus. That’s what we are all hoping for. Our staying in the mainstream of the country depends on them.”

He insists, repeatedly, that he is “not concerned”. But as he goes to say goodbye he pauses in the door frame. For a moment he is silent. “Let us pray for the Muslims of Gorakhpur,” he finally says. “Even if Yogi is harming Muslims in other parts of the country, he won’t do anything to Muslims in Gorakhpur. Of that I’m very sure.”