From bloodshed in the Bible and the combat of the Crusades to today's terrorist attacks in the United States and suicide bombings in the Mideast, violence in the name of religion has a long history.
Sept. 11 has focused new attention on the phenomenon, however, with experts citing a greater need for understanding the forces behind it.
"Religion is the ideology of protest,'' said Mark Juergensmeyer, professor at the University of California, Santa Barbara, and author of Terror in the Mind of God.
In the book, Juergensmeyer found that the fall of the Soviet Union and the end of the Cold War helped create new grounds for conflicts around the world.
"People are sanctioned to kill in defense of country and defense of religion,'' Juergensmeyer, said. "For some entities, the fight is no longer my form of government against yours. It is my religion and my beliefs against yours.''
Just 20 years ago, he noted, the U.S. State Department listing of international terrorist groups included only a handful of religious groups. But by 1998, he said, more than half of the terror organizations listed by Secretary of State Madeleine Albright were religious in nature.
Although some of these groups commit terror in the name of Islamic fundamentalism, that is just one type of religious-based violence—alongside Protestant- Roman Catholic conflict in Northern Ireland, cult attacks on Tokyo's subway system in 1995, and the targeting of abortion clinics and their doctors in the United States.
Researchers say the victimizers often see themselves as victims.
"People who get into this usually feel deprived in some way--politically, financially or psychologically. They have the feeling something is missing from their lives and others have it,'' said Jessica Stern, a Harvard researcher who has profiled terrorists.
"Extremists have certain things in common across religious lines, but it seems that Islamic extremists are more successful in mobilizing their young men to their cause than other faiths,'' Stern said. "I think part of that is because they seem to be as much or more focused on the next life as they are on the present life.''
Asma Afsaruddin, a professor of Arabic and Islamic Studies at the University of Notre Dame, said enormous anger is a driving force behind religious violence. "Add to that a sense of moral righteousness and a sense of having a monopoly on truth. What I'm basically describing is a fanatic.''
People who use violence for religious ends often carry a sense of righteous indignation, even if they do not voice these strong opinions to others, said Rona Fields, a Washington-based psychologist who has studied terrorists and their organizations since 1970.
"They see their role in life as an avenging angel,'' Fields said.
Political goals also are a crucial catalyst when violence and religion mix. "It's the perceived political injustices that fuel the spiral of violence,'' Afsaruddin said.
Small groups of extremists who use violence in the name of religion usually are led by people with strong personalities and excellent communication skills, said Jay Demerath, a sociology professor at the University of Massachusetts.
"It's a tight circle of social support, where they get someone to be so dedicated to a cause it becomes all-consuming,'' said Demerath, author of Crossing the Gods--World Religions and Worldly Politics.
Demerath agreed that a political goal--a desire "to change the state structure"--is often a factor for those committing violence in the name of religion. But there's more, he said.
"In most of the cases, the bombers are recruited and they find that gratifying. For the first time, they are very important people.''