The Church of Scientology claims:
It is a fact that unless one begins to handle aberrations built up in past lives, he doesn’t progress.
Past lives is not a dogma in Scientology, but generally Scientologists, during their auditing, experience a past life and then know for themselves that they have lived before.
– Source: Does Scientology believe in reincarnation or past lives? Scientology Catechism FAQ
Researcher John Weldon explains Scientology’s philosophy, including its belief in past lives:
The basic tenets of Scientology result from an eclectic mixture of Eastern philosophy and the personal research of Hubbard into a variety of disciplines, as well as the ”data” uncovered from ”auditing.” Auditing is Scientology’s ”counseling” or extensive examination of the present life and ”past lives” of the ”preclear,” or initiate. In one of its many definitions, Hubbard has described Scientology as ”the Western Anglicized continuance of many earlier forms of wisdom.” These include the Vedas, Taoism, Buddhism, Judaism, Gnosticism and early Greek civilization; and the teachings of Jesus, Nietzsche, and Freud. According to Hubbard, ”Scientology has accomplished the goal of religion expressed in all Man’s written history, the freeing of the soul by wisdom.”
Scientology divides the mind into two components — the analytic and the reactive, roughly parallel to the conscious or rational mind and unconscious or irrational mind. Experiences of extreme shock, pain, or unconsciousness cause ”engrams,” or sensory impressions, to be recorded in the reactive mind. These mental pictures are, in turn, the cause of our emotional and even many physical problems today. They can be dislodged only through Scientology.
While these memory pictures are perfectly recorded, they lay dormant in the brain until restimulated by a similar incident. When restimulated, they cause conditioned, stimulus-response behavior which is counterproductive to one’s well-being. Thus, when the brain sees a similar situation to a past threatening experience — even though it is not now a threat to survival, it responds as if it were, producing a form of inappropriate and self-defeating behavior. For example, a boy falls out of a tree just as a red car passes by and is knocked unconscious. Later, even as a man, red cars (even red things) may restimulate the episode in various ways and cause irrational reactions. This man may thus refuse to ride in a red car and may even get ill or dizzy when confronted with the possibility.
In this sense, we are all more or less conditioned beings — ”machines” that simply respond to their operator (i.e., the reactive mind). Scientology believes this restimulation is fairly automatic. In other words, we are not free beings: we are slaves of an ”aberrated” (reactive) mind. Scientology maintains that through Dianetic and/or Scientology therapy, we can be directly exposed to our engrams, ”erase” them, and become ”clear,” or in control of our behavior (”at cause”) rather than at the mercy of a damaged reactive mind (”at effect”).
Unfortunately, Scientology informs us, through reincarnation we have all been accumulating engrams for trillions of years. Thus, to resolve hidden engrams, not only must the initiate be mentally whisked back to reexperience the damaging events of this life, but of many past lives as well.
According to Scientology, each person is really a thetan, an immortal spirit who has been so damaged by engrams that he has forgotten he is immortal and even forgotten he is a thetan. Thetans have absolute control over their bodies, but, sadly, they think they are bodies (a terrible fate) and hence are bound by the MEST (matter, energy, space, time) universe. Each time a body dies, the thetan must enter another body, but this brings with it all its trillions of years’ accumulation of engrams. Thetans thus are no longer free, but are in bondage to the material universe. Scientology claims it can free the thetan.
– Source: Is Scientology compatible with Christianity? John Weldon. See footnotes at original article.