The stark cliffs of the Zagros Mountains on the Iran-Iraq border, and the dusty hills and plains that lie between those mountains and the city of Mosul, might seem an unlikely location for paradise. Yet Christians living here in past centuries believed that a local river called the Great Zab had once flowed from Adam and Eve's garden. Patriarchs of the Christian Assyrian Church of the East living on its banks once signed off their letters with the salutation, "From my cell by the river of Eden". These days the Patriarch's letters are sent from a less romantic spot: 7201 North Ashland Boulevard, Chicago. Successive waves of persecution have driven out the leaders of this ancient, prestigious and little-known church -- including Mar Dinkha IV, the present and 120th Patriarch of Babylon, who was consecrated in Ealing, west London, and is based in the United States. As for the Great Zab, this summer it ended up as the de facto border between Kurdish forces on its southern side and the so-called Islamic State (IS) to its north. From being the garden of Adam and Eve, Iraq has become the land of Cain and Abel. Yet even its melancholy recent history can remind us that the religious conflict that scars the modern Middle East is far from inevitable. In 1987, Christians in Iraq numbered 1.4 million. Since then, the country's population has doubled but its Christian community has declined to 400,000. Many of these people are now internally displaced because of IS, a Sunni Muslim militant movement that drove them from their homes in August 2014 in its effort to establish an Islamic "caliphate". The former Christian inhabitants of Mosul and the surrounding towns are now refugees in the semi-autonomous Kurdistan Region nearby, protected from the summer heat and winter snow only by UN-provided tents erected in local churchyards. "We were given just a few hours to leave Mosul," one of the refugees told me last summer in the sun-scorched streets of Erbil, capital of the Kurdistan Region. "We fled to Qaraqosh [a Christian town just east of Mosul] and then Islamic State came there, too, and we had to flee Qaraqosh." He was one of a group of men sitting by the road under the shade of a wall; this was how they spent their days, because the tents in which they slept provided barely enough room at night and were left during the day to the women and children. Unable to afford cigarettes, the men passed the time chatting and then at mealtimes headed to the church-organised canteen that handed out free food. Not that there was much help on offer for them, the refugee said. Nobody cared about them and any aid that was supposed to reach them was being siphoned off. "Nothing like this," he said dolefully, "has ever happened before." Except, it has. The Christians of Iraq have endured worse and survived. Their community in Baghdad was battered in the Mongol invasions of the 13th century and destroyed by the central Asian warlord Tamerlane in 1402: he gave orders that the only things that should be left standing in Baghdad were hospitals and mosques. Those who survived Tamerlane fled north into the Zagros Mountains, joining others who lived in a band of territory along the northern edge of Iraq and Syria and the southern edge of Turkey. There, in 1915-16, they were caught up in the massacres inflicted by the Ottoman authorities on the Armenians. An estimated 250,000 Syrian and Iraqi Christians were slaughtered, or starved, or died of exposure during forced marches. Others fled to Iran, from where they were in turn displaced to Iraq. Their abandoned homes can still be seen in the now-tranquil towns of southern Turkey. Unlike with either of these historical horrors, Islamic State's ability -- though not its ambition -- to spread murder and oppression among Iraq's Christians has proved limited. Since its initial successes, IS has been unable to make further inroads into Kurdistan, or fulfil its vainglorious boast that it would capture Baghdad. Muslims, not Christians, have borne the brunt of its brutality. Nonetheless it may manage to achieve what Tamerlane and the Ottomans did not: the final extinction of the Christian community in Iraq. Deprived of their ancient heartland in and around Mosul, Iraq's Christians are now divided between Baghdad and Kurdistan. Baghdad houses roughly 100,000 of them; but the very government of Iraq is run by religious partisans from the Shia Muslim sect. A Yazidi activist who tried urging Iraqi parliamentarians in Baghdad to save his people (the Yazidis, who preserve ancient pre-Islamic traditions, are even more vulnerable than the Christians) told me that the lawmakers' response was that his people could save themselves best by converting to Islam. The Kurdistan authorities are keener to keep their Christian residents, and apparently their leader, President Masoud Barzani, has discussed a proposal to build new Christian towns within the region's borders to accommodate the refugees from Mosul. But Kurdistan cannot provide work for all the refugees, and because of its oil economy and the high demand for housing locally, the cost of living there is much higher than in Mosul. For "90 per cent" of the Christian refugees, as more than one of them told me, there is no solution except emigration from the Middle East. Who are these Christians of Iraq and where did they come from? And how have they come to be on the verge of disappearance from their own homeland? The Church of the East -- which is now split between those who follow Mar Dinkha IV, and others who accept the Pope in Rome as their ultimate spiritual leader -- was originally the community of Christians who lived in the Persian empire. Most of them were related to the people of Syria and they spoke a version of Aramaic, which they wrote with Syriac characters. Their form of Christianity evolved in ways that marked them out from their western counterparts. When looking to expand and spread their beliefs, they looked not west towards Europe, but east, towards India and China. They were the first to introduce Christianity to the Chinese and the Mongols and to this day the Mongolians use an alphabet based on Syriac characters. Genghis Khan's daughters-in-law were Christian and eventually the Church of the East had a Mongolian patriarch. A network of monasteries and churches spread eastwards from Baghdad to Beijing, encompassing a bishopric of Tibet and another in Kashgar, a Silk Route city in western China. Much of this happened while the patriarch of the Church of the East was living under Muslim rule, following the Arab conquests of the 630s AD. Along with followers of other pre-Islamic religions, Baghdadi Christians were used by the Muslim Arabs as decipherers of Greek science and occasionally as ministers and advisers. The patriarch was permitted to debate theology with the Muslim caliph. And yet, subsequently, the fortunes of Christians in the Middle East declined. Perhaps it was inevitable, as their numbers dwindled and their power waned, that they would be exploited by rapacious governments. This was exacerbated by conflicts between Christian and Muslim states, including the Crusades. However, it also coincided with the collapse of the Arab caliphate and the rise of others -- such as Turks and Mongols -- who had the zeal of new converts, saw religion as the binding force that legitimised their own rule and were not attracted by the rationalist tendencies that had once been popular in Baghdad. In an Arab world ravaged by conflict and ruled by outsiders, few intellectuals remained who could resist populist dogmatic conservatism. A similar change has happened in the Arab world in the past half-century. In the 19th century, as the Ottoman empire decayed, resurgent nationalism went hand in hand with religious emancipation. The rulers of Egypt, for example, wanted to promote an Egyptian identity in which Christians, Muslims and Jews could all participate. Between 1860 and 1930 Egypt had three Christian prime ministers. To be sure, the ruler was always a Muslim, because Egypt was a monarchy; but let's remember that Britain to this day has never had a Catholic prime minister and that Spain only revoked the 1492 expulsion of its Jews in 1968. So, parts of the mostly Muslim Middle East were heading towards religious equality faster than Europe. As nationalism spread across the Arab world, other Christians took prominent positions. One, Michel Aflaq, was a founder of the Ba'ath Party, which ruled Iraq and still rules part of Syria. Christians led two Palestinian nationalist movements and some played a part in the Kurdish national movement. Others were leading communists, attracted by an ideology that also offered equality to religious minorities. Even as late as 2003 Iraq still had a Christian, Tariq Aziz, as its deputy prime minister. (He is in prison, enduring desperate conditions.) This is not, by the way, an endorsement of any of those entities, which could be ruthless to those who opposed them. But they were at least movements that were open to any who wanted to join them. In the Middle East over the past few decades, by contrast, the most popular movements have been religious. Islamic zealots came to power in Iran's revolution in 1979, the postwar Iraqi elections of 2005 and Egypt's presidential elections in 2012. Religious observance has risen, too. In the 1950s attendance at the yearly Ashura procession in Karbala, Iraq, was so thin that a senior cleric felt the need to launch a movement to rekindle religious sentiment. In 2014, two million people attended the festival. Meanwhile, the clerics' political movement, called the Islamic Dawa Party, has taken over the government of Iraq. Why the religious revival? In my years in the Arab world working as a diplomat, I often debated this question with Arab friends, almost all of them believing Muslims, who nonetheless felt alienated by the rise of fundamentalist Islam. Is it caused by poverty, or the lack of democracy, or the failure of the rule of law? No: the revival has happened also among Muslims in the west, and in relatively democratic and prosperous countries such as Turkey, as well as autocratic ones. (Indeed, some of the poorest of Muslims -- in remote parts of Afghanistan, for instance -- are among the least radical.) Is it because of colonial injustices, sometimes described as "Muslim grievances"? To some extent: yet these grievances were once seen as ethnic, or class-related, rather than religious; and often the victims of these colonial injustices, most obviously in Palestine, included Christians as well as Muslims. Is it because the conflict between Shias and Sunnis has heightened people's sense of their religious identity? Yes, but that only raises a further question of why the conflict happened along religious lines in the first place. The more fundamental reasons are fivefold. First, money: formerly provided to left-wing movements by the Soviet Union, now plentifully available from Iran for Shia revolutionaries and from the Arab Gulf for those who are most hostile to Iran, many of them Sunni Islamists. Second, the defeat of nationalist governments by Israel in the 1967 war and the subsequent failure of secular authorities and movements to capture the public imagination and loyalty. In Egypt, according to recent Gallup polling, religious authorities (Christian or Muslim) command the respect of 92 per cent of the population, far ahead of any other institution. Third, the connivance of western governments -- and Israel, in fact -- in the rise of Islamist movements in the 1970s, when they were seen as a safe alternative to nationalists and communists. Fourth, the weakness of the education system in many Arab states, whose heavy focus on rote learning reinforces dogmatic literalism, and which often does little to educate students about cultures and religions other than Islam. The last reason is perhaps even more significant. The Lebanese writer Amin Maalouf has written about what he calls the "intense religiosity of the urban migrant", who sees religion as a way to protect himself and his family from the temptations of urban life. The rise in religiosity in the Muslim world has coincided with mass migration to the cities. It has also coincided with globalisation, which has undermined indigenous Arab cultures, leaving religion as the sole clear criterion of identity and the focus of national pride. Perhaps we should not be surprised to discover how many of IS's supporters had previously appeared to be thoroughly westernised: this is perhaps the very reason they feel such a passionate need to recapture their sense of being separate and different. Although the rise of religious exclusionism and violence is a large part of the reason for Christian migration, it also happens for more ordinary reasons: economics, for example. The precipitate shrinkage of the Iraqi Christian community after 1987 did not begin with the 2003 war, nor with the rise to power of Islamist parties in 2005, nor even the 2014 massacres. It began instead with the sanctions imposed on Iraq after Saddam Hussein's 1990 invasion of Kuwait, which prompted middle-class Christians to seek refuge in the US, where many had relatives, following previous waves of persecution. There are still more than ten million non-Muslims in the Arab world, the great majority of whom are Christians. And even if almost all of them leave within the next half-century, they will survive in exile, at least for a few generations, though transplanted to western countries devoid of any of their ancient shrines and monasteries. Hundreds of thousands of Iraqi Christians now live in the sprawling suburbs of metropolitan Detroit in the US. They have tried hard to hold on to their heritage -- largely marrying among themselves, and even maintaining their Aramaic language among their children and grandchildren. The Middle East is greatly poorer for their absence. After the failure of their attempt to hold violently on to power in Lebanon, the Christians have become an increasingly neutral group politically. Their presence is often a liberalising factor, because, as a people exempt from Islamic law, they are a reason why states cannot seek to impose sharia on all their citizens (a reason, of course, why they are targeted by extremists). Without the Christians, the region will be even less liberal and more monochrome, and will risk becoming more isolated. The Middle East would also lose a part of the heritage and history that all its people, Muslim or Christian, have in common. For the Christian communities have preserved parts of their nations' heritage: Aramaic in Iraq, pharaonic hymns in Egypt. Their diversity (there are innumerable sects) reflects the region's history, each sect tracing its origin to the political developments of one era or another. The schools that Christians run in the Middle East, open to Muslims, have educated generations of Arabs. There is one further and wider point that the survival of Christians and other non-Muslim minorities makes. By their continuity and sheer existence in the Middle East, these communities remind us that the Islamic world has not always been the bloody tragedy that it is today. It has seen much violence over the centuries, true; but it has also been strengthened by its own diversity, and coexistence between the various religions. It was at its best and most flourishing when it treated diversity as a strength and not a weakness. We all lose if that lesson is forgotten.

Gerard Russell is a former British and UN diplomat. He is the author of "Heirs to Forgotten Kingdoms: Journeys into the Disappearing Religions of the Middle East", newly published by Simon and Schuster.