The Future of an Illusion, published by Sigmund Freud in 1927, extends his psychoanalytic theory of religion into a direct argument that religious belief functions as a collective illusion, in Freud's technical sense meaning a belief motivated primarily by wish fulfillment rather than by evidence, whether or not that belief happens to be objectively true or false. Freud argues that religious doctrines, particularly belief in a protective, providential God, answer to the same underlying psychological needs that make an individual child depend on a protecting father: comfort in the face of a frightening and uncontrollable natural world, and consolation for the inevitability of death and the demands civilization places on instinctual desire.
Freud is careful to distinguish an illusion in his specific sense from a simple error or delusion, defining it instead as a belief whose real driving force is the wish behind it, regardless of the belief's actual truth or falsehood, a definitional move central to how he frames religion as psychologically explicable without that explanation alone settling whether any particular religious claim is true.
The book's title plays deliberately on two senses of the word illusion, since Freud is asking not only what future religious belief itself has but what future a civilization built on illusion can expect, framing psychological honesty about religion's origins as a precondition for humanity's continued intellectual and social maturation rather than a merely destructive critique.