In Somalia, al-Shabab, which slaughtered scores of people at a Kenyan shopping mall in September, has reportedly vowed to kill every Somali Christian.

In northern Nigeria, Boko Haram has butchered thousands of Christians, as well as Muslims they consider inadequately ideological - such as those seeking an education.

Four of every five acts of religious discrimination in the world today are directed against Christians, according to the Germany-based International Society for Human Rights. The secular US think tank the Pew Forum says Christians face harassment or oppression in 139 nations, nearly three-quarters of all the countries on earth.

It is not just Muslims, who themselves often face horrendous persecution, who attack Christians. In the Indian state of Orissa, Hindu nationalists attacked Christians in a vicious pogrom in 2008, killing 500, injuring thousands with machetes, and leaving 50,000 homeless. A nun was raped and paraded naked through the streets, watched by police who arrested no one.

In Burma, Bhutan, Nepal and Sri Lanka, Buddhist militants have murdered Christians, Muslims and Hindus. In 2010 the Burmese military attacked Christian minorities from helicopters, reportedly killing thousands.

These cases are horrific, certainly, but surely they are disconnected and accidental acts of cruelty and violence? Not so, rights observers say: they are all part of the biggest human rights challenge now facing the globe - religious intolerance - and also part of a largely unobserved global war on Christians. Things may be worse now for more Christians than at any time in history, including under the Roman Empire.

"War" does not mean a unified campaign directed by a single co-ordinating mind. But it is no exaggeration, Vatican analyst John Allen argues in his new book, The Global War on Christians, because it represents a "massive, worldwide pattern of violence and oppression directed against a specific group of people, often explicitly understood by its perpetrators as part of a broader cultural and spiritual struggle". If we are not honest enough to call it a war, we will not face it with the necessary urgency, he says.

What is happening? Why are Christians especially at risk, and why are Western governments, media and churches so reluctant to acknowledge it, let alone act? And, as some observers suggest, is religious persecution heading back to the West?

Religion is often only one factor in this violence, part of a combustible cocktail of racial, ethnic, economic and linguistic motives, but increasingly - such as with the rising tide of puritanical Muslim Salafists - it is the main or only reason. And in the countries where the problem is most severe, persecution has accelerated and deepened in the past two years.

The international Catholic charity Aid to the Church in Need last week launched its 191-page report Persecuted and Forgotten, challenging the international community's willingness to stand up for religious freedom.

The report calls the flight of Christians from the Middle East an exodus of almost biblical proportions. "Incidents of persecution are now apparently relentless and worsening: churches being burnt, Christians under pressure to convert, mob violence against Christian homes, abduction and rape of Christian girls, anti-Christian propaganda in the media and from government, discrimination in schools and the workplace."

Long-time religious liberty analyst and advocate Liz Kendal says when she began monitoring religious violence 15 years ago, "I was reporting on an attack here or there, usually a militant who came in and attacked a missionary. Now it's pogroms where people massacre their neighbours with machetes and with impunity". Kendal is the Melbourne-based advocacy director of Christian Faith and Freedom.

This is a frightening new feature, that neighbours join or lead the brutality. "One of the disturbing things about Syria is not just all the al-Qaeda-linked groups, but that local Muslims welcome them. They want their Christian neighbours to leave," Kendal says.

Persecution can be a nebulous term. Both Christians and Muslims in the West have used it to refer to non-life-threatening discrimination. American scholar Charles Tieszen's definition is a good one: any unjust action of mild to intense levels of hostility, directed at people belonging to a religion resulting in varying levels of harm, in which the victim's religious identification is the main motive.

Todd Johnson, of Gordon Conwell's Centre for the Study of Global Christianity, estimates 70 million Christians have died for their faith, 45 million of them in the 20th century.

John Allen notes that "this boom in religious violence is still very much a growth industry. Christians today are by some order of magnitude the most persecuted religious body on the planet," suffering not just martyrdom but all forms of intimidation and oppression in record numbers.

The US Commission on International Religious Freedom, which monitors religious persecution and names the worst offenders in an annual report, listed 16 nations guilty of "heinous and systematic" offences in its 2012 report.

Only one group is under attack in all 16 nations: Christians. (The countries are Burma, China, Egypt, Eritrea, Iran, Iraq, Nigeria, North Korea, Pakistan, Saudi Arabia, Sudan, Tajikistan, Turkey, Turkmenistan, Uzbekistan and Vietnam.)

Open Doors lists 25 countries as most hazardous, 18 of them Muslim-majority nations - six in Asia, seven in Africa, eight in the Middle East, and four in the former Soviet empire. As Allen notes, this shows that it really is a global war.

The Middle East, the birthplace of Christianity, may soon be emptied of its adherents, and of other religious minorities. In Iraq, which had 1.5 million Christians before the first Gulf War, the total is now possibly as little as a 10th of that. Most have fled, but unnumbered thousands have been killed.

Muslims also suffer greatly - in Buddhist Burma and Thailand, in Hindu India and communist China, and in Muslim countries where their particular form is a minority. Hindus are persecuted in Buddhist countries, such as Sri Lanka. Iranian authorities, brutal against Christians, are even more vicious when it comes to Baha'is. Persecution seems an equal-opportunity affair.

Nor are Christians immune from perpetrating violence, as the world has seen in Rwanda, the Congo and Yugoslavia in the past 20 years. Yet when it comes to victims, they are well out in front. Why?

World Evangelical Alliance spokesman Thomas Schirrmacher says a number of factors combine. Christianity is much the biggest religion, so its numbers are likely to be large, and it is experiencing enormous growth in dangerous places where it makes established groups feel threatened. Religious nationalists tend to identify Christianity with Western colonialism. Christians, supported by better international networks, also tend to be more outspoken in advocating rights and democracy and in opposing corruption.

Dictators fear that Christians do not give them the undivided allegiance they demand (think North Korea, China or Vietnam), while some commentators even suggest Christians help bring suffering on themselves because of their willingness to turn the other cheek - militant Muslims might be more wary if they didn't have impunity, if Christians too adopted suicide bombing.

Why, 1700 years after the Edict of Milan, in which Constantine decreed religious tolerance in the Roman Empire, is religious intolerance so savage? A number of cross currents have come together, including rising religious nationalism, Islamic fundamentalism driven particularly by Saudi Arabia's petrodollars, victory for Islamists against Russia in Afghanistan, which sent the jihadis back to their various homes with ambitions entrenched, and the loss of American political influence after the global financial crisis.

This has been encouraged by a shameful apathy or denial by First World leaders. When it comes to secular politics, the victims are too Christian to matter much to the left, who are much more comfortable bashing the doubtless legitimate but comparatively minor target of Israel. And they are too brown or too foreign to matter much to most on the right.

Secularists also tend to think of Christians as the oppressor, not the oppressed. When they picture persecution, they turn to history: the crusades, the Inquisition, Europe's savage 17th century religious wars, and colonial exploitation. But, as John Allen observes: "Today we do not live on the pages of a Dan Brown potboiler, in which Christians are dispatching mad assassins to settle historical scores. Instead, they're the ones fleeing assassins others have dispatched."

He also cites two sets of "blinders". Christians in the West can overstate the struggles they face from an increasingly post-Christian state, which diminishes sympathy for the Christians in real danger. Second, Western powerbrokers tend to underestimate the role religion plays in persecution in the Third World, its consistency as a driver.

Liz Kendal says there was a brief period when the US made a difference through its religious freedom bill. Introduced in 1998, it worked well for a decade, but collapsed with the global financial crisis in 2008 when the US economic clout "evaporated overnight and religious liberty was affected immediately, especially in China and Iran", she says.

"Now the gloves are off. Persecution with impunity is the order of the day and no one can stop it. America could threaten sanctions, and things would settle down, but those days are over. "

Kendal is scathing about Western churches, saying they often deliberately avert their eyes. "The Western church is so happy having a nice time in celebratory worship, they don't want the burden of this knowledge (of what is happening to their brethren). Pastors feel under pressure to have their congregations leave the church feeling upbeat."

She says the churches have to stop expecting political solutions. "The cavalry is not going to come over the hill, and it's not where the church's faith should be anyway."

Her pessimism runs deep. Not only is religious persecution unstoppable in Islamic and other Third World countries, but it is on the way in the West, if in a different form, she says.

Where Christian social conservatism was once mainstream, she predicts Christians will face jail and other sanctions if they do not toe the fast-changing secular line on such issues as condoning homosexuality and same-sex marriage.

Cardinal Francis George, the Catholic Archbishop of Chicago, made a similar prediction, noting in 2010: "I expect to die in bed, my successor will die in prison, and his successor will die a martyr in the public square."

And why does mainstream Western media miss the big picture? "That's the million-dollar question, and I don't know," Kendal replies. She suggests it is a combination of ignorance by journalists about the historical and political context of persecution and a political correctness that will not allow them to criticise Muslims for fear of being labelled racist or Islamophobic. "It's just too hot to handle," she says.

"Turn on your TV and there is a young BBC reporter in Syria saying 'these freedom fighters are fighting for democracy'. And behind him are bushy-bearded jihadists waving a black flag and shouting 'Allahu akbar' [God is great], fresh from cutting throats."

In Burma, Kendal says, Western journalists believe the regime's talk of reform and don't realise Aung San Suu Kyi has been silenced, or the religious hatred that is directed against ethnic minorities. In Sudan, the Islamic regime is running a declared jihad against the African Christians, who are sitting on the last of the country's oil. "It's genocide taking place before our eyes, and we're not talking about it."

Paul Marshall, author of Blind Spot - When Journalists Don't Get Religion, thinks another factor is that so few journalists are Christian. Thus they tend to think that religion doesn't have any intellectual content, it is merely feelings and emotion, so it's not worth the effort to learn about it.

Marshall, a senior fellow at the Hudson Institute for Religious Freedom in Washington, says the churches, in turn, are not very good at talking to journalists. It's easy, too, to overlook that opponents such as Osama bin Laden have had a coherent, intelligent view of the world, even if we disagree with it.

Meanwhile suffering Christians might find scant consolation in the knowledge they were warned - Jesus says, in the Gospel of John: "In the world ye shall have tribulation: but be of good cheer; I have overcome the world."

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