"As soon as man was capable of conceiving the idea of sin, he had recourse to psychic concealment-- or, to put it in analytical language, repressions arose. Anything that is concealed is a secret. The maintenance of secrets acts like a psychic poison which alienates their possessor from the community. In small doses, this poison may actually be a priceless remedy, even an essential preliminary to the differentiation of the individual. This is so much the case that, even on a primitive level, man has felt an irresistible need to invent secrets; their possession saves him from dissolving in the unconsciousness of mere community life, and thus from a fatal psychic injury. As is well known, the many ancient mystery cults with their secret rituals served this instinct for differentiation. Even the Christian sacraments were looked upon as mysteries in the early Church, and, as in the case of baptism, were celebrated in private apartments and only referred to under a veil of allegory.
However beneficial a secret shared with several persons may be, a merely private secret has destructive effect. It resembles a burden of guilt which cuts off the unfortunate possessor from communion with his fellow-beings. Yet if we are conscious of what we conceal, the harm done is decidedly less than if we do not know what we are repressing-- or even that we have repressions at all. In the latter case we not merely keep a content consciously private, but we conceal it even from ourselves. It then splits off from consciousness as an independent complex, where it can be neither corrected nor interfered with by the conscious mind. The complex is thus an autonomous portion of the psyche which, as experience has shown, develops a peculiar fantasy-life of its own. What we call fantasy is simply spontaneous psychic activity; and it wells up whenever the repressive action of the conscious mind relaxes or ceases altogether..."
C.G. Jung, from the essay, Problems of Psychotherapy.