Religious fanaticism, or religious extremism, is a pejorative designation used to indicate uncritical zeal or obsessive enthusiasm that is related to one's own, or one's group's, devotion to a religion – a form of human fanaticism that could otherwise be expressed in one's other involvements and participation, including employment, role, and partisan affinities. Historically, the term was applied in Christian antiquity to denigrate non-Christian religions, and subsequently acquired its current usage with the Age of Enlightenment.
Lloyd Steffen cites several features associated with religious fanaticism or extremism:
Spiritual needs: Human beings have a spiritual longing for understanding and meaning, and given the mystery of existence, that spiritual quest can only be fulfilled through some kind of relationship with ultimacy, whether or not that takes the form as a "transcendent other". Religion has power to meet this need for meaning and transcendent relationship.
Attractiveness: It presents itself in such a way that those who find their way into it come to express themselves in ways consistent with the particular vision of ultimacy at the heart of this religious form.
A 'live' option: It is present to the moral consciousness as a live option that addresses spiritual need and satisfies human longing for meaning, power, and belonging.
Christianity and violence
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. Ellens says, "When Christianity came to power in the empire of Constantine, it proceeded to almost 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".