Fanaticism is the correlate to spiritual self-surrender” it shows the anxiety which it was supposed to conquer, by attacking with disproportionate violence those who disagree and who demonstrate by their disagreement elements in the spiritual life of the fanatic which he must suppress in himself. Because he must suppress them in himself he must suppress them in others. His anxiety forces him to persecute dissenters. The weakness of the fanatic is that those whom he fights have a secret hold upon him; and to this weakness he and his group finally succumb.
Paul Tillich, The Courage to Be