The persecution of Christians in the Middle East is a "once-in-a-thousand-year-crisis", but they are refusing to back down in the face of Islamic State, US author Johnnie Moore has said.

In an exclusive interview with Christian Today, Moore – whose book 'Defying ISIS', published this weekend, is based on exhaustive research – said that though ISIS militants have systematically targeted numerous religious minorities, including Shiite Muslims, "it is absolutely true that they [ISIS] have a particular interest in eliminating the Christian communities. It's overt, it's not hidden and it's not an exaggerated crisis."

The front page of the October edition of ISIS' online propaganda magazine Dabiq featured the Islamic State flag depicted as flying from the obelisk in St Peter's Square. In an accompanying article, the group renewed its threat to "conquer" Rome, and urged Muslims to kill "every crusader possible...wherever they can be found".

Just two weeks ago, militants targeted Assyrian Christians in the Khabour region, taking more than 200 hostage and killing at least 30. Jihadists then bulldozed the ancient Assyrian city of Nimrud, a site of such historical significance that an expert likened it to the Egyptian pyramids, and later also destroyed the city of Hatra – another World Heritage Site considered to be one of the most important in the world.

The Assyrians have been targeted "solely because they are Christians", Moore said. "And it's sort of akin to what the Nazis did – they've squeezed them out...The atrocities against women and children are incomprehensible."

But despite the relentless persecution, the title of Moore's book was inspired by the response of Christians to their situation. He visited refugee camps in Irbil, Iraq, in October last year, where he was astounded by the faithfulness of those who had lost everything. The camps were disastrous, he says, with families absolutely desperate, living in makeshift tents clearly not robust enough to survive the winter.

"They were living in this awful, awful place, and yet on tent after tent, I saw how they had taken crosses and put them at the entrance...I saw people with crosses tattooed on their arms, and women with crosses around their necks," Moore recalled.

"ALL WE HAVE LEFT IS GOD"

"That cross is what nearly cost them their lives, and yet the one thing they weren't going to do was put that cross down. Person after person I met said, 'We've lost our homes, our future, everything. All we have left is God,' and that's all they needed. I just remember thinking: why are so few of us willing to live for what they are so willing to die for?"

Moore, who is in regular contact with top US officials about the crisis, believes the only response to ISIS can be one of forgiveness and love. A narrative that stokes interreligious conflict only serves to empower the extremists, he said.

"It's the best of faith that defeats the worst of religion...Christianity is a religion of love, and it's not a religion of crusades as ISIS would have it defined," he said. He's not alone. Following the beheading of 21 Coptic Christians in Libya last month, the General Bishop of the Coptic Orthodox Church in the UK immediately expressed forgiveness. "I think as Christians that's our mandate," Bishop Angaelos told Christian Today. "It's what we do. I don't see it as being difficult."

"That type of love is more powerful than hate," Moore said. "There's that verse that now makes sense to me in Song of Solomon where it says 'love is more powerful than death'. And that's the thing – all the hate in the world can't defeat the faith of some of these [Christians]."

Inter-faith unity is more important than ever, he added. Christians and Muslims must work together to unpick the narrative spread by ISIS that the two are incompatible, and "if there has ever been a time for us to stop caricaturing and characterising one another in overly simplistic and extremist ways, it's now."

And yet, though ISIS appears to be gaining ground – now controlling an area of land larger than the UK – Moore believes there is still hope. The first page of his book features a verse from Romans 16: "Soon the God of peace will crush Satan underneath your feet."

"This isn't the first time that evil men have beheaded Christians. The second martyr, James, was beheaded, Paul was beheaded and Peter was crucified. Innumerable Christians in history have faced this same threat," he explained.

When ISIS destroyed Jonah's tomb last year, they inadvertently uncovered the ruins of an ancient Byzantine church that had remained hidden for centuries. The Archbishop of Mosul, who had also lost everything when militants overran the city last June, told Moore at the time: "The gates of hell will never prevail against the Church".

"And he was right," Moore said. "I absolutely believe that when it's all said and done the God of peace will crush Satan under his feet."