"The man in whom intolerance reaches the point of flaring up, the point of fanaticism, is like a jealous person: he sees everywhere only one thing—only the treason, the betrayal, only the breaking of fidelity to this single thing. He becomes suspicious and mistrustful. He discovers everywhere conspiracies against his beloved idea, against the object of his faith and love. And just like the jealous person, he is very difficult to bring back to reality."
"Obsessed with a maniac pursuit, the fanatic sees all around the snares of the devil. In fact, however, he is the one who always persecutes, torments, and executes. The man in the grip of a persecution mania, the man who senses enemies all around him, is a very dangerous being: he always becomes the persecutor... A man who's obsessed with the searching out and detection of heresies, intent upon the excommunicating and pursuing of heretics, is a man long since accused and judged by Christ."
"To the fanatic, the devil seems terrible and strong. He believes in the devil more so than he believes in God."
"The fanatic acts with force. He excommunicates, imprisons, and executes. But in essence he is weak, not strong. He is smothered by fear, and his consciousness is terribly narrow, for he believes less in God than the tolerant do. In a certain sense it might be said that a fanatic faith is a weakness of faith, a lack of faith... Fanatic intolerance involves always a profound lack of faith in humanity, in the image of God within humanity, a lack of faith in the power of truth—that is, in the final end, a lack of faith in God... Having allowed himself to come under the obsessive grip of the idea of a worldwide peril and worldwide conspiracy of Masons, of Jews, of Jesuits, of Bolsheviks, or of an occult society of killers, the fanatic ceases to believe in the power of God, in the power of truth, and he trusts only in his own coercions, cruelties, and murderings."
"The fanatic always has need of an enemy. He always needs someone to execute."
"Fanaticism does not permit the co-existence of various ideas and world-outlooks. There exists only the enemy. The hostile powers blend together and present themselves as a single enemy... For the fanatic there does not exist a manifold world. This is a man obsessed by one thing. He has a merciless and malevolent attitude towards all and everything except for this one thing."
"The egocentrism of the fanatic... expresses itself in this: he does not see the human person; he remains inattentive to the person's human journey; he is unable to establish any sort of relationship to the world of persons, to the living, concrete human world."
"Pathological hatred for heresy is obsession with an 'idea,' which is set higher than humanity. But all the orthodox doctrines of the world are nothing in comparison with 'the least of these.' Human beings are the image and likeness of God. Every system of ideas, however, is only the product of human thought or thoughtlessness. Human beings are not to be saved or damned by cleaving to some sort of system of ideas... A believing, an unselfish, an intellectual man can become a fanatic and commit the greatest of cruelties. To devote oneself without reservations to God or to an idea, substituting it for God, whilst ignoring human beings, is to transform the person into a means and a weapon for the glory of God or for the realization of the idea. To do that means to become a fanatic—wild-eyed and even a monster."
"The fanatic of an orthodoxy can be an extreme adherent of the principle of authority. But he always imperceptibly identifies the authority with himself and is never subject to any sort of authority in disagreement with him... Such authority in practice never vexes its fanatical adherents; it vexes others, their opponents, and does violence to them. In essence, no one subjects themselves to authority if they consider it not to be in accord with their understanding of truth."
"The guardians of 'orthodoxy' are the ones that distort the truth most of all and are afraid of it."
"Behind the unmasking of heretics there is always concealed a sinful lust for domination, a will to power... People esteeming themselves preeminently 'orthodox' whilst denouncing heretics—that is, those who think differently—love to declare that they are defending truth, and they set truth up higher than freedom... But the man fanatical over some sort of idea, like a person who would save himself alone, cannot be said to seek the truth. The search for truth presupposes freedom. Truth is not external to freedom, truth is bestowed only by freedom. Outside of freedom there is only that which is useful, but not truth; there is only the interests of power. The fanatic of 'orthodoxy' seeks for power, not for truth."
"The pathos of an 'orthodoxy' fed by fanaticism has nothing in common with the pathos of truth; it is actually contrary to it. Such an 'orthodoxy' forms itself around themes of salvation and perdition, and such 'orthodox' persons are themselves frightened and frighten others. Truth however does not know fear."
excerpted from this 1937 essay: "Fanaticism, Orthodoxy, and Truth" (lightly reworked for readability)