Doubt is based on man's separation from the whole of reality, on his lack of universal participation, on the isolation of his individual self. <...>He flees from his freedom of asking and answering for himself to a situation in which no further questions can be asked and the answers to previous questions are imposed on him authoritatively. In order to avoid the risk of asking and doubting he surrenders the right to ask and to doubt. <...>. He "escapes from his freedom" (Fromm) in order to escape the anxiety of meaninglessness <...> Meaning is saved, but the self is sacrificed. And since the conquest of doubt was a matter of sacrifice, the sacrifice of the freedom of the self, it leaves a mark on the regained certitude: a fanatical self-assertiveness. 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. 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.
From The Courage to Be