By John Blake, CNN

(CNN) - An angry outburst at a mosque. The posting of a suspicious YouTube video. A friendship with a shadowy imam.

Those were just some of the signs that Tamerlan Tsarnaev, accused of masterminding the Boston Marathon bombings, had adopted a virulent strain of Islam that led to the deaths of four people and injury of more than 260.

But how else can you tell that someone’s religious beliefs have crossed the line? The answer may not be as simple you think, according to scholars who study all brands of religious extremism. The line between good and evil religion is thin, they say, and it’s easy to make self-righteous assumptions.

“When it’s something we like, we say it’s commitment to an idea; when it’s something we don’t like, we say it’s blind obedience,” said Douglas Jacobsen, a theology professor at Messiah College in Pennsylvania.

Yet there are ways to tell that a person’s faith has drifted into fanaticism if you know what to look and listen for, say scholars who have studied some of history’s most horrific cases of religious violence.

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“There are a lot of warning signs all around us, but we usually learn about them after a Jim Jones or a David Koresh,” said Charles Kimball, author of “When Religion Becomes Evil.”

Here are four warning signs:

1. I know the truth, and you don’t.

On the morning of July 29, 1994, the Rev. Paul Hill walked up to John Britton outside an abortion clinic in Pensacola, Florida, and shot the doctor to death. Hill was part of a Christian extremist group called the Army of God, which taught that abortion was legalized murder.

Hill’s actions were motivated by a claim that virtually all religions espouse: We have the truth that others lack.

Those claims can turn deadly when they become absolute and there is no room for interpretation, Kimball says.

“Absolute claims can quickly move into a justification of violence against someone who rejects that claim,” Kimball said. “It’s often a short step.”

Healthy religions acknowledge that sincere people can disagree about even basic truths, Kimball says.

The history of religion is filled with examples of truths that were once considered beyond questioning but are no longer accepted by all followers: inerrancy of sacred scripture, for example, or the subjugation of women and sanctioning of slavery.

If someone like Hall believes that they know God’s truth and they cannot be wrong, watch out, Kimball says.

“Authentic religious truth claims are never as inflexible as zealous adherents insist,” he writes in “When Religion Becomes Evil.”

Yet there’s a flip side to warnings about claiming absolute truth: Much of religion couldn’t exist without them, scholars say.

Many of history’s greatest religious figures – Moses, Jesus, the Prophet Mohammed – all believed that they had discovered some truth, scholars say.

Ordinary people inflamed with a sense of self-righteousness have made the same claim and done good throughout history, says Carl Raschke, a theology professor at the University of Denver in Colorado.

The Protestant Reformation was sparked by an angry German monk who thought he had the truth, Raschke says.

“Martin Luther’s disgust at the worldliness of the papacy in the early 1500s inspired him to become a radical revolutionary whose ideas overturned the entire political structure in Europe,” Raschke said.

So how do you tell the difference between the healthy claims of absolute truth and the deadly? Scholars say to look at the results: When people start hurting others in the name of their religious truth, they’ve crossed the line.

2. Beware the charismatic leader.

It was one of the deadliest terrorist attacks in Japanese history. In March 1995, a religious sect called Aum Shinrikyo released a deadly nerve gas in a Tokyo subway station, leaving 12 people dead and 5,000 injured.

Two months later, Japanese police found Shoko Asahara, the sect’s founder, hiding in a room filled with cash and gold bars. Kimball, who tells the story of the sect in “When Religion Becomes Evil,” says Asahara had poisoned the minds of his followers years before.

Asahara demanded unquestioned devotion from members of his sect and isolated followers in communities where they were told that they no longer needed to think for themselves, Kimball says.

Any religion that limits the intellectual freedom of its followers, he says, has become dangerous. “When you start to get individuals who are the sole interpreters of truth, you get people who follow them blindly."

Charismatic leaders, though, often don’t start off being cruel. Jim Jones, who led the mass suicide of his followers in South America, was a gifted speaker who built an interracial church in San Francisco that did much good in the community. Few people at the beginning of his ministry could predict what he would become.

As time went on, though, his charisma turned cruel as he tolerated no questions to his authority and became delusional.

“Charismatic leadership is important, but in healthy religions, there’s always a process where questions are encouraged,” Kimball said.

Weaning followers away from corrupt charismatic leaders and bad religion can take years, but it can be done if one knows how to speak their language, says Ed Husain, senior fellow at the Council on Foreign Relations in New York.

Countries such as Saudi Arabia and Egypt will often deploy imams to reach out to young men in prison who have adopted “Islamism,” or extreme forms of Islam sanctioning violence against civilians, says Husain, who has written about Muslim extremism.

These Muslim clerics know the Quran better than the extremists and can use their knowledge to reach extremists in a place that logic and outsiders cannot penetrate, Husain said.

“The antidote to extremism is religion itself,” Husain said. “The problem is not to take Islam out of the debate but to use Islam to counter Islamism.”

3. The end is near.

In 1970, an unknown pastor from Texas wrote a book called “The Late, Great Planet Earth.” The book, which linked biblical prophecy with political events like Israel’s victory in the 1967 Six-Day War, predicted the imminent return of an antichrist and the end of the world.

Author Hal Lindsey’s book has sold an estimated 15 million copies and spawned a genre of books like the “Left Behind” series. Many people are fascinated by the idea that the heavens will open soon because the end is near.

That end-times theology can turn lethal, though, when a follower decides that he or she will speed up that end-time by conducting some dramatic or violent act, says John Alverson, chairman of the theology department at Carlow University in Pittsburgh.

“A religious terrorist mistakenly believes that God has ordained or called him or her to establish the will of God on Earth now, not gradually and not according to the slow and finicky free will of other humans,” Alverson said.

Yet this impulse to see God’s intervention in human affairs now and not in some distant future can also be good, he says.

There are vibrant religious communities that teach that political and economic injustice must be addressed now. Liberation theology, for example, was a movement among pastors and theologians in Latin America that called for justice for the poor now, not in some future apocalyptic event, Alverson says.

“Hope is a good breakfast but not much of a supper,” Alverson said. “We can’t just live on the hope that justice will happen; we have to actually experience justice from time to time so that our hope can continue.”

4. The end justifies the means.

It was one of the biggest scandals the Roman Catholic Church ever faced, and the repercussions are still being felt today.

In January 2002, the Boston Globe published a story about Father John Geoghan, a priest who had been moved around various parishes after Catholic leaders learned that he had abused children. It was later revealed that Catholic officials had quietly paid at least $10 million to settle lawsuits against Geoghan.

Kimball says the Catholic scandal revealed another sign that a faith has turned toxic: Religious figures start justifying doing something wrong for a higher good.

“The common theme was trying to protect the integrity of the church,” Kimball said of some Catholic leaders who covered up the crimes. “You get all of these rationalizations that we can’t let this scandal bring the whole church down, so we have to pay off this family and send the priests off to rehab.”

Religion is supposed to be a force for good. Still, it’s common that everyone from suicide bombers to venal church figures finds ways to justify their behavior in the name of some higher good.

Those rationalizations are so pervasive that religious movements that avoid them stand out, scholars say.

Jacobsen, the theology professor from Messiah College, cited the civil rights movement. The Rev. Martin Luther King Jr. and his fellow activists renounced violence, even as they were attacked and sometimes murdered.

“They were willing to lay down their lives for what they believed in, but what’s incredible is, they practiced not retaliating when they suffered violence,” he said. “Those people really believed that God created everyone equal, and they were committed to the point of death.”

In some ways, it’s easy to say we would never adopt a form of religion that’s evil. But when we use the word “evil” to describe those who kill in the name of their faith, we’re already mimicking what we condemn, Jacobsen says.

In his new book, “No Longer Invisible: Religion in University Education,” Jacobson writes that calling a religion evil is dangerous because “bad or wrong actions can be corrected, but typically evil needs to be destroyed.”

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“To label someone or something as evil is to demonize it, putting it in a category of otherness where the rules of normal life do not apply, where the end often justifies almost any means,” Jacobson writes.

And when we do that, we don’t have to read about radical imams or look at angry YouTube videos to see how easy it is for someone to drift toward religious extremism, he says.

We need only look at ourselves.