In any human endeavor, some fraction of its practitioners will be motivated to pursue that activity with such concentrated focus and unalloyed passion that it will consume them utterly. One has to look no further than individuals who feel compelled to devote their lives to becoming concert pianists, say, or climbing Mount Everest. For some, the province of the extreme holds an allure that’s irresistible. And a certain percentage of such fanatics will inevitably fixate on matters of the spirit.

The zealot may be outwardly motivated by the anticipation of a great reward at the other end — wealth, fame, eternal salvation — but the real recompense is probably the obsession itself. This is no less true for the religious fanatic than for the fanatical pianist or fanatical mountain climber. As a result of his (or her) infatuation, existence overflows with purpose. Ambiguity vanishes from the fanatic’s worldview; a narcissistic sense of self-assurance displaces all doubt. A delicious rage quickens his pulse, fueled by the sins and shortcomings of lesser mortals, who are soiling the world wherever he looks. His perspective narrows until the last remnants of proportion are shed from his life. Through immoderation, he experiences something akin to rapture.

Although the far territory of the extreme can exert an intoxicating pull on susceptible individuals of all bents, extremism seems to be especially prevalent among those inclined by temperament or upbringing toward religious pursuits. Faith is the very antithesis of reason, injudiciousness a crucial component of spiritual devotion. And when religious fanaticism supplants ratiocination, all bets are suddenly off. Anything can happen. Absolutely anything. Common sense is no match for the voice of God . . .

— Jon Krakauer, Under the Banner of Heaven