The Courage To Be, Paul Tillich, 1952. from pages 47, 48.

The anxiety of emptiness is aroused by the threat of nonbeing to the special contents of the spiritual life. A belief breaks down through external events or inner processes: one is cut off from creative participation in a sphere of culture, one feels frustrated about something which one had passionately affirmed, one is driven from devotion to one object of devotion to another and again on to another, because the meaning of each of them vanishes and the creative eros is transformed into indifference or aversion.

Everything is tried and nothing satisfies. The contents of the tradition, however excellent, however praised, however loved once, lose their power to give content today. And present culture is even less able to provide the content. Anxiously one turns away from all concrete contents and looks for an ultimate meaning, only to discover that it was precisely the loss of a spiritual center which took away the meaning from the special contents of the spiritual life. But a spiritual center cannot be produced intentionally, and the attempt to produce it only produces deeper anxiety.

The anxiety of emptiness drives us to the abyss of meaninglessness.
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... 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 tried 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 own 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.