More than 20,000 foreign fighters from 80 countries have joined the ranks of ISIS and other extremist groups in Iraq and Syria. Somewhat surprisingly, of that total, almost 4,000 fighters have come from Western Europe.

Why would so many leave relatively comfortable lives in the West to take up jihad?

Religious extremists often believe they are waging a divine battle for good against evil and have historically justified their actions – no matter how violent or grotesque – as appropriate, divine acts in the service of God. Their opponents therefore become not just ideological opponents, but amoral enemies of God. The defense of God’s message against such enemies in turn enables extremists to justify and rationalize all manner of actions that appear cruel or bizarre to outsiders.

Islamic extremism is grabbing headlines today for its widespread brutality, but religious-based extremism is far from exclusive to Islam. For example, Jewish extremist Baruch Goldstein committed a reprehensible massacre of 29 Muslim worshippers in a Hebron mosque in 1994. In addition, violent Jewish Israeli nationalists have committed so-called price-tag attacks on Muslim businesses, houses of worship, and private property. In one such recent attack, members of a Jewish anti-Arab group set fire to a bilingual Hebrew-Arabic school in Jerusalem. An Orthodox rabbi from Teaneck, N.J., made international headlines in November 2014 with a blog post stating, “Arabs who dwell in the land of Israel are the enemy in that war and must be vanquished.”

One manifestation of Christian extremist violence took place in 1994 when the Rev. Paul Hill killed Dr. John Britton and his bodyguard outside a Florida abortion clinic. Before his 2003 execution, Hill said he expected “a great reward in Heaven.”

The Army of God, a Virginia-based anti-abortion group, has claimed responsibility for the bombings of abortion clinics in Georgia and Alabama, while praising the murders of abortion doctors – though it did not claim direct responsibility for those acts. Its website invites visitors to send thank-you notes to Scott Roeder, convicted for the 2009 murder of Dr. George Tiller, medical director of a Kansas abortion clinic.

Christian Pastor Terry Jones of Florida issued a worldwide call in 2010 to burn copies of the Koran on September 11, which he dubbed “International Burn a Koran Day.” The Southern Poverty Law Center lists Jones as part of the “Anti-Muslim Inner Circle.”

All of these people were motivated by what they believed to be a divine duty. Clearly, even as mainstream religious leaders seek to condemn religious-based extremism, the justification of violence as a holy endeavor remains an enduring phenomenon across religions.