BAGHDAD -- Priest Wasseem Sabeeh was halfway through Sunday Mass, in Our Lady of Salvation in Baghdad, when an explosion shook the church. Suddenly men with guns yelling Islamic prayers burst into the church. They fired at the priests, congregants, even murals of Mary and the saints. Some parishioners screamed and ran out. Sabeeh, 27, and another priest, Thaer Saadallah, 32, hastily directed dozens of others into a room near the altar, then turned to plead with the men in suicide vests to stop the killing. They shot Sabeeh at point-blank range, then shot Saddallah in the face. He fell on the steps of the altar, his vestments stained with blood. "They were the best people," said Withaina Hadi, 49, a Muslim woman waiting at a health clinic run by Christians at St. George's, an Anglican church on the other side of the Tigris River. A terrorist group that calls itself the Islamic State of Iraq, which is linked to al-Qaeda, claimed responsibility for the attack. The group seeks the establishment of harsh Islamic law in Iraq and says all Iraqi Christians are targets for jihad. Horrified by the threat, many Christians are telling their religious leaders that they plan to go. "They will definitely leave," said Faiz Bashir, St. George's curate. "We hope it's not the end of Christians in Iraq, but if things get worse, if there are attacks on the churches and killing on the streets, this will be certain." The four-hour ordeal Oct. 31 at Our Lady of Salvation, which left 50 people dead, was only the bloodiest of numerous terrorist attacks targeting Iraq's Christian communities in the past year. The terrorists, who have also killed hundreds of Muslims in bombings in the past several months, have stepped up the targeting of Christians in recent weeks. On Wednesday, 11 roadside bombs went off within an hour in three predominantly Christian areas of central Baghdad. Four blasts hit houses belonging to Christians, and two mortar rounds struck Christian enclaves in the predominantly Sunni Muslim neighborhood of Dora. Two bombs planted in deserted Christian homes in western Baghdad destroyed two houses. On Tuesday, a series of bombs hit three empty houses belonging to Christians in western Baghdad. Christians have been living in Iraq since long before it was called Iraq and centuries before Mohammed was born. Ancient Assyrians converted to Christianity during the first century A.D. Back then, Iraq was part of what was known as Mesopotamia, and it would not be until the eighth century A.D. that Islam arrived. Even today, tombs of Old Testament Jewish prophets such as Ezekiel and Daniel dot Iraq. Standing in his sunlit, yellow-brick church, built in 1936, British clergyman Andrew White said St. George's has increased security and will have service on future Sundays. He is not expecting many people to come. But if he preaches, he said, he will remind congregants of Jesus' words to disciples who were worried about how Jewish leaders would react to their newfound faith: "They will put you out of the synagogue; in fact, the time is coming when anyone who kills you will think they are offering a service to God," according to the Gospel of John. "When the incident happened, I lived this verse, I saw it happening," White said. Human Rights Watch says the number of Christians in Iraq had fallen to about 675,000 in 2008 from 1 million at the time of the U.S.-led invasion in 2003. The Catholic Church in Iraq says there are 1.5 million Christians in Iraq, 1 million fewer than 2003. Many Christians have fled Iraq for Syria, Jordan or Egypt. Many of those who remain in the country are leaving their homes behind for safer areas such as the Kurdish-held north. Emma Hovasapien, 50, a Christian who works near Our Lady of Salvation, said that her neighbors have begun selling their furniture. She spent five years abroad but had struggled to find work and came back this year to find an Iraq worse than she had left it. "This is my country, but how long can we hold on -- that is the question," she said. On Tuesday, Iraqi Prime Minister Nouri al-Maliki visited the church where the massacre happened and vowed to protect Christians. The Iraqi government says it wants to stop the exodus. "It is very sad and unfortunate that the Christian Iraqis are leaving," said Kamil Amin, spokesman of the Ministry of Human Rights. "Their presence is part of the historical legacy of Iraq." Iraq's top Catholic prelate, Chaldean Cardinal Emmanuel III Delly, has encouraged the remaining Christians to stay in the country and called on the authorities for more protection. Amin criticized France and other countries for offering asylum to Christian families. He said such offers are encouraging a mass departure and said the Iraqi government is committed to protecting Christians. Many Muslims who live near Our Lady of Salvation say they have no dispute with Christians and are anguished over the deaths of the churchgoers. "Some of them were our neighbors," said Ekhlas Abduljabar, 32, a Muslim woman who said she often had the priests bless her children with holy water if they were sick. "We never had a sectarian dispute," she said. "This is their country as well. They have been here forever."