Religious fanaticism is fanaticism related to a person's, or a group's, devotion to a religion. However, religious fanaticism is a subjective evaluation defined by the culture context that is performing the evaluation. What constitutes fanaticism in another's behavior or belief is determined by the core assumptions of the one doing the evaluation. As such, there is currently no constant academic standard for what defines a fanatical religious position.
In his book, Holy War, Just War, Lloyd Steffen says, "[Religious] fanaticism . . . invokes the idea of ultimacy, and its presence in religious life is undeniable."  He goes on to say, "[Religious] fanatics are persons who attach to some object an ultimate valuation and then attend to that overvalued object with what is recognizable as a kind of religious devotion." 
Steffen gives several features associated with religious fanaticism or extremism. Calling it "the demonic", he says:
There are some who think that certain scriptures can influence fanatic and even violent behaviors. In Volume 3 of his book, The Destructive Power of Religion: Violence in Judaism, Christianity, and Islam, J. Harold Ellens indicates a few possibly fanatic stories in The Bible.
It was abusive for Jesus to abandon the healed man to the assaults of the Pharisees, whose psychology was . . . taking the role of the scolding parents and putting the healed man down into the role of the naughty child. . . .The gamesmanship of the healed man does not obviate the fact that Jesus abused him, exploited him for his own purposes, abandoned him to significant persecution, and only thereafter, when all the damage was done, embraced him in a redemptive way. 
[Christ] walked into the temple . . . trying to find a place of tranquility in which to pray and an audience with whom he could discuss the coming kingdom of God. All he could see was the hated priests in their formalistic rituals. All he could hear was the bawling of the cattle. All he could smell was the odors of the stable. . . .He cracked. He picked up a riding crop or bullwhip and . . . abuse[d] those most available, expending his long-anguished anger, his weariness with the spiritual mediocrity of human life. 
Ever since Christianity was brought to power, 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. Yet Christians, in persecutions of other religions, in wars about religion, and in wars of conquest, have perhaps been more violent than members of any other religion except Islam".  The start of Christian fanatic rule came with the Roman Emperor Constantine I. 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 continued into the Middle Ages with the Crusades. These wars were attempts by the Christians, sanctioned by the Pope, to reclaim the Holy Land from the Muslims. 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 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. 
In addition, John Edwards, in a review of an article called "Was the Spanish Inquisition Truthful?" says, "Ferdinand and Isabella's Inquisition . . . repressed . . . the natural yearnings of . . . Jews who had converted to Christianity . . . after the attacks mounted against numerous Jewish communities in the early summer of 1391." 
Meriedith De Silva also published an article "Christian Fanaticism in Sri Lanka" which explained how Christian extremists are taking over Sri Lanka. She explained how Christianity made a violent introduction in Sri Lanka almost 500 years ago. Unlike Buddhism, Islam, and Hinduism, Christianity arrived in Sri Lanka using force. Native priests were killed, temples looted and destroyed, all in the name of Jesus. Those who refused to convert religion were harassed or even killed.
In Sri Lanka today, Christian militants are going house to house in an attempt to convert everyone to Christianity and are using whatever means necessary. Christian extremists in Sri Lanka are destroying religious harmony in order to claim Christianity as the sole religion in Sri Lanka.
During the 19th century, most Christian nations have adopted the principle of separation between church and state. Religious fanaticism is since an internal problem of the Christian churches or merely a personal (psychological) problem. However, this is not so in most modern Muslim countries (except, for example, Turkey under Ataturk.)
Charles Selengut explains in his book "Sacred Fury" that Christianly calls on the fact that they offer their body to god, just like many other religions do. Christians make many sacrifices also, as Charles Selegnut states, "[Christianity] places great value upon avoiding bodily temptations and accepting pain and sufferings as sacred activity and promises religious rewards to those who do so." He also describes the Christian "martyrdom" which is more literally referring to all who offer their life and well-being for the cause of God and religion. 
In the eyes of some, some of the violent actions of the Israelites in the times of the Old Testament were fanatical in nature. Steffen describes the Holy War waged by the Israelites, along with the God of the Old Testament, Yahweh. He describes this "divinely sanctioned use of force and violence", or herem, saying:
In the herem, Yahweh seeks general destruction, as in the conquest of Canaan (Josh. 6:17-18), or even total annihilation, as in God's command to King Saul to include in the slaughter of Israel's enemies ‘man and woman, child and infant, ox and sheep, camel and donkey' (1 Sam. 15:3; 22:19). 
As Steffen says, this war bordered on omnicide.  The Israelites dealt with neighboring cities in a way that some would deem fanatical also. Paul N. Anderson says that "nearby cities were to be destroyed completely. . . .If they are not, they will corrupt the Israelites with their religion and detestable practices and will cause Israel to fall away from God."  In addition, he mentions that the people were told to obey Yahweh's commands completely, and, for the most part, the people obeyed. The only king that defied Yahweh during the Holy War was Saul, who preserved the flocks of the people the Israelites attacked because he didn't see a point in wasting them.  This, as the scriptures say, angered Yahweh, thus proving, in Steffen's eyes, that Yahweh demanded absolute obedience from his chosen people.
Perhaps Islam has come to be the most publicized religion with members who display fanatic tendencies. Ever since Osama bin Laden's fatwa in 1998, the world has known about radical jihad. 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". 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. Osama bin Laden's fatwa illustrates the goal of fanatic jihad: "In compliance with God's order, we issue the following fatwa to all Muslims: the ruling to kill the Americans and their allies—civilians and military—is an individual duty for every Muslim who can do it".  Fanatic jihadists' beliefs, as Ellens says, stem from a feeling of inferiority to Western civilization. He says:
Because of its sense of inferiority in power and its sense of arrogant superiority in spiritual and religious quality, this militant form of Islam feels thoroughly justified in resorting to the most vicious forms of violent assault on its identified enemy. America is the perceived source and center of its problems. 
Thomas Farr, in an essay titled "Islam's Way to Freedom", goes further, saying 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)
This disregard for others and rampant use of violence is markedly different than 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, 2001 bombings of the World Trade Center. 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".