Symbols cannot be produced intentionally ... They grow out of the individual or collective unconscious and cannot function without being accepted by the unconscious dimension of our being. Symbols which have an especially social function, as political and religious symbols, are created or at least accepted by the collective unconscious of the group in which they appear.
...Symbols cannot be invented. Like living beings, they grow and die. They grow when the situation is ripe for them, and they die when the situation changes. The symbol of the "king" grew in a special period of history, and it died in most parts of the world in our period. Symbols do not grow because people are longing for them, and they do not die because of scientific or practical criticism. They die because they can no longer produce response in a group where they originally found expression.
--The Essential Tillich, p. 42-43