Religious fanaticism is uncritical zeal or with an obsessive enthusiasm related to one's own, or one's group's, devotion to a religion - a form of human fanaticism which could otherwise be expressed in one's other involvements and participation, including employment, role, and partisan affinities.
Steffen gives several features associated with religious fanaticism or extremism:
Ever since Christianity was established, some of those in authority have sought to expand and control the church, often through the fanatical use of force. Grant Shafer says, "Jesus of Nazareth is best known as a preacher of nonviolence.  The start of Christian fanatic rule came with the Roman Emperor Constantine I as Catholicism. Ellens says, "When Christianity came to power in the empire of Constantine, it proceeded almost to viciously repress all non-Christians and all Christians who did not line up with official Orthodox ideology, policy, and practice". An example of Christians who didn't line up with Orthodox ideology is the Donatists, who "refused to accept repentant clergy who had formerly given way to apostasy when persecuted". Fanatic Christian activity, as Catholicism, continued into the Middle Ages with the Crusades. These wars were attempts by the Catholics, sanctioned by the Pope, to reclaim the Holy Land from the Muslims. However many Catholics see the crusades as a just war in self-defense. Charles Selengut, in his book Sacred Fury: Understanding Religious Violence, said:
The Crusades were very much holy wars waged to maintain Christianity's theological and social control. On their way to conquering the Holy Land from the Muslims by force of arms, the crusaders destroyed dozens of Jewish communities and killed thousands because the Jews would not accept the Christian faith. Jews had to be killed in the religious campaign because their very existence challenged the sole truth espoused by the Christian Church.
Shafer adds that, "When the crusaders captured Jerusalem in 1099, they killed Muslims, Jews, and native Christians indiscriminately". Another prominent form of fanaticism according to some came a few centuries later with the Spanish Inquisition. The Inquisition was the monarchy's way of making sure their people stayed within Catholic Christianity. Selengut said, "The inquisitions were attempts at self-protection and targeted primarily "internal enemies" of the church". The driving force of the Inquisition was the Inquisitors, who were responsible for spreading the truth of Christianity. Selengut continues, saying:
The inquisitors generally saw themselves as educators helping people maintain correct beliefs by pointing out errors in knowledge and judgment... Punishment and death came only to those who refused to admit their errors ... during the Spanish Inquisitions of the fifteenth century, the clear distinction between confession and innocence and remaining in error became muddled.... The investigators had to invent all sorts of techniques, including torture, to ascertain whether ... new converts' beliefs were genuine.
Extremism within Islam goes back to the 7th century to the Kharijites. From their essentially political position, they developed extreme doctrines that set them apart from both mainstream Sunni and Shi?a Muslims. The Kharijites were particularly noted for adopting a radical approach to Takfir, whereby they declared other Muslims to be unbelievers and therefore deemed them worthy of death.
Sayyid Qutb, a figurehead of the Muslim Brotherhood in Egypt, was influential in promoting a Pan-Islamist ideology in the 1960s. When Qutb was executed by the Egyptian government, Ayman al-Zawahiri formed the organization Egyptian Islamic Jihad to replace the government with an Islamic state that would reflect Qutb's ideas for Islamic revival. The Qutbist ideology has been influential in jihadist movements that seek to overthrow secular governments, and Qutb's books have been frequently been cited by Osama bin Laden and Anwar al-Awlaki.
Since Osama bin Laden's fatwa in 1998, radical jihad has increasingly become an internationally recognized term. Bin Laden's concept, though, is very different from the actual meaning of the term. In the religious context, jihad most nearly means "working urgently for a certain godly objective, generally a positive one". The word jihad in Arabic means 'struggle'. The struggle can be a struggle of implementing the Islamic values in daily activities, a struggle with others to counter arguments against Islam, or self-defense when physically attacked because of belief in Islam. According to Steffen, there are portions of the Qur'an where military jihad is used. As Steffen says, though, "Jihad in these uses is always defensive. Not only does 'jihad' not endorse acts of military aggression, but 'jihad' is invoked in Qur'anic passages to indicate how uses of force are always subject to restraint and qualification". This kind of jihad differs greatly from the kind most commonly discussed today.
Thomas Farr, in an essay titled "Islam's Way to Freedom", says that, "Even though most Muslims reject violence, the extremists' use of sacred texts lends their actions authenticity and recruiting power". (Freedom 24) He goes on to say, "The radicals insist that their central claim - God's desire for Islam's triumph - requires no interpretation. According to them, true Muslims will pursue it by any means necessary, including dissimulation, civil coercion, and the killing of innocents". (Freedom 24)
According to certain observers this disregard for others and rampant use of violence is markedly different from the peaceful message that jihad is meant to employ. Although fanatic jihadists have committed many terroristic acts throughout the world, perhaps the best known is the September 11 attacks. According to Ellens, the al-Qaeda members who took part in the terrorist attacks did so out of their belief that, by doing it, they would "enact a devastating blow against the evil of secularized and non-Muslim America. They were cleansing this world, God's temple".