Emptiness and loss of meaning are expressions of the threat of nonbeing to the spiritual life. This threat is implied in man's finitude and actualized by man's estrangement. It can be described in terms of doubt, its creative and its destructive function in man's spiritual life. Man is able to ask because he is separated from, while participating in, what he is asking about. In every question an element of doubt, the awareness of not having, is implied. In systematic questioning systematic doubt is effective; e.g. of the Cartesian type. This element of doubt is a condition of all spiritual life. The threat to spiritual life is not doubt as an element but the total doubt. If the awareness of not having has swallowed the awareness of having, doubt has ceased to be methodological asking and has become existential despair. On the way to this situation the spiritual life tries to maintain itself as long as possible by clinging to affirmations which are not yet undercut, be they traditions, autonomous convictions, or emotional preferences. And if it is impossible to remove the doubt, one courageously accepts it without surrendering one's convictions. One takes the risk of going astray and the anxiety of this risk upon oneself. In this way one avoids the extreme situation till it becomes unavoidable and the despair of truth becomes complete.
Then man tries another way out: 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. So he tries to break out of this situation, to identify himself with something transindividual, to surrender his separation and self-relatedness. 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 surrenders himself in order to save his spiritual life. He "escapes from his freedom" (Fromm) in order to escape the anxiety of meaninglessness. Now he is no longer lonely, not in existential doubt, not in despair. He "participates" and affirms by participation the contents of his spiritual life. 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. 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")