Iraqi security officers stand guard outside of the Virgin Mary Chaldean church in the Karrada neighborhood of the capital Baghdad on Dec 21, 2014. (Sabah Arar/AFP/Getty Images)

A choir dressed in crimson robes sang ancient hymns below a Christmas star strung with fairy lights at a recent service in the Iraqi capital, the heavy scent of incense hanging in the air. But the season here has a somber edge, and the priest has a serious message for his congregation: Stay.

Just a year ago, an Advent service at St. George’s Chaldean Catholic Church would have drawn 300 to 400 worshipers, says the Rev. Miyassir al-Mokhlasee. But now only around 75 people are scattered across its pews.

Ringed by concrete blast walls and police checkpoints, the church has seen its congregation shrink for the past decade. The instability and violence following the U.S.-led invasion in 2003 have driven many Christians out of the country. The nation’s Christian population has plummeted from more than a million to what community leaders estimate is less than 400,000 today.

Conquests by extremists from the Islamic State, known for their cutthroat brutality and intolerance for other religions, have delivered another blow to Christians in their historic heartland.

For the first time in well more than a millennium, the plains of Nineveh and its provincial capital of Mosul have been virtually emptied of Christians. Islamic State fighters, who control the northern region, had ordered Christian residents to convert, pay a tax for keeping their faith or face execution.

Iraqi Christians light candles inside a shrine in the grounds of Mazar Mar Eillia (Mar Elia) Catholic Church, that has now become home to hundreds of Iraqi Christians who were forced to flee their homes as the Islamic State advanced earlier this year, on Dec. 12, 2014 in Erbil, Iraq. (Matt Cardy/Getty Images)

Religious sites such as Mosul’s tomb of Jonah, the ancient figure whose story of being swallowed by a whale is told in the Christian and Muslim holy books, have been blasted apart. Other minorities such as the ethnic Shabak and

the long-persecuted Yazidis have faced similar mass displacement and killings in the once richly diverse region.

The reverberations are being felt further afield in the capital, where families are packing to leave — hoping to gain a new life overseas.

Ayad Imad, 22, a Catholic resident of the middle-class Baghdad neighborhood of Zayouna and sales manager for an international cigarette firm, is one of them. This Christmas will be his last in Iraq, he said.

His parents have sold their house and cars. As soon as his father finishes a round of medical treatment, the family will travel to Turkey, where they plan to register with the United Nations High Commissioner for Refugee’s asylum program, with the hope of being resettled in North America, Europe or Australia.

They might have to wait a long time before they reach one of those destinations. But Imad doesn’t care. He has spent years lobbying his family to emigrate from Iraq, but his father had not wanted to leave his elderly parents behind.

“At first my father insisted we stay,” he said. “But my father’s had a job, a career. My grandfather is an old man, he’s lived. Now it’s my turn to live my life, and there’s no future here.”

It was the Islamic State offensive this summer — in which the extremists overran Mosul — that finally convinced Imad’s father that it was time to leave.

Many who pack up and go don’t tell their friends and neighbors, virtually disappearing overnight. In the precarious security environment, families fear that they will become a target for kidnappers in the days before they leave the country, as word spreads that they are cash-rich after having sold assets such as houses and cars.

“If it stays this way, we will shrink to nothing,” said Father Mokhlasee, sinking his head into his hands. “We believe that God wants us here for diversity in the region. Unfortunately, people are afraid of the future, and they are leaving.”

One of Iraq’s most senior Christian religious figures, Chaldean Catholic Patriarch Louis Sako, has accused the United States of being “indirectly responsible” for the exodus of one of the world’s most ancient Christian communities, pointing to the chaos caused by the 2003 invasion.

In the sectarian warfare and lawlessness that followed the outbreak of war, Christians were often caught in the crossfire or targeted for kidnapping. In Imad’s neighborhood, Christian shops have been attacked for selling alcohol, and many have closed down.

In July, lines formed daily outside the French Embassy in Baghdad after Paris announced it was ready to facilitate asylum for displaced Christians. Iraqi Christian communities in the United States have called on the Obama administration to do the same.

But Younadam Kanna, a prominent Christian parliamentarian, maintains that such programs are counterproductive.

“It’s a disaster,” he said. “Violence and discrimination and corruption are kicking us out, then others are pulling us out. The international community is encouraging Christians to leave. This is destroying our community here.”

It is not the first time Father Mokhlasee’s church has witnessed an exodus. In 2010, when an attack on Baghdad’s Syriac Catholic Cathedral during evening Mass left 58 dead, the Iraqi capital’s Christian community was shaken. The priest said few of those who left his parish after that incident returned.

He fears his church will not be able to survive the loss of many more parishioners.

“We are becoming fewer in number,” he said in his Advent sermon. “We ask God that we can keep our churches, keep our country. We have a message that people should stay in this country.”

But afterward, as families milled around in the church’s courtyard, he said he feared his message is falling on deaf ears.

“Even my family is leaving,” the priest said. His brother with his wife and four children are planning to move to Jordan after losing their farm in the Nineveh town of Qaraqosh when it was overrun this summer. “He’s looking for a future for his children.”