PHILADELPHIA — Does Tom Cruise really believe that thousands of years ago, in a previous life, he — and all humankind — became earthlings by landing here on a Noah's Ark-like spaceship?

Was John Travolta's 2000 turkey of a movie, "Battlefield Earth," actually a coded way to spread Scientology's doctrines? (If so, does that mean humankind's worst enemies look like Spock — but with dreadlocks?)

Scientology is once again making headlines, what with the announcement that Cruise's fiancee, Katie Holmes, is "converting" to the religion (it has no official conversion process), and with Cruise's recent outburst against psychiatry on the "Today" show. Not only is psychiatry a pseudoscience, Cruise said, it's a tool to control the masses.

So just what do Scientologists believe? And why is it so attractive to Cruise, Travolta, Isaac Hayes, Juliette Lewis, Kirstie Alley and the eight million other people the group claims as members in 154 countries?

The most widely known aspects of Scientology — its association with celebrities and its attack on psychiatry — are great clues. First, one of the promises its texts implicitly make is that Scientology can give anyone the ability to succeed in any venture by cultivating the same self-confidence, charisma and power we so envy in our celebs. And it does it through therapy.

To sociologist David G. Bromley of Virginia Commonwealth University, Scientology is a "quasi-religious therapy" that resembles Freudian "depth psychology" while also drawing upon Buddhism, Hinduism, and the ancient, heretical offshoot of Christianity known as gnosticism.

The overtly therapeutic process, which Scientology founder L. Ron Hubbard introduced in 1950, is a technique aimed at self-mastery.

Hubbard maintained that stressful events cause the "rational mind" to shut down, allowing the fear-based "reactive mind" to process the events as traumas.

As Hubbard explains in his book "Dianetics," each trauma is stored in the mind as "engrams," packets of negatively charged memories that accumulate over time, further impeding self-determination. Because he believed in reincarnation, Hubbard held that — like karma in Hinduism — engrams pile up from previous lives, forming a formidable obstacle to self-actualization.

To end this cycle, a Scientologist goes through a carefully scripted form of therapy called auditing intended to — eventually — erase his emotional baggage, making him a "Clear." (Enamored of tech lingo, Hubbard took the term from the button on a calculator that deletes previous operations.)

In an auditing session, the client plays a word-association game with the "auditor," who explores words that remind him of some past trauma — bringing up its engram. By discussing each of these past events, the client's engrams eventually lose their negative charge.

One controversy around Scientology is over its cost. Reportedly, an auditing session costs beginners no more than $50, but people end up spending hundreds of thousands of dollars to reach Clear.

Scientology says the self-confidence that comes from this process arms one to be more successful in daily life. As Bromley writes in a study, "Religion as Therapy, Therapy as Religion: The Church of Scientology as a Quasi-Religious Therapy," the organization says Clears are well-suited to be managers and can heal psychosomatic illnesses and even physical ailments.

Although a good deal of Scientology's theology is kept secret, the adept, now called an "Operating Thetan" (OT), reportedly undertakes an eight-level ascension leading to the awareness that his true essence is not his body but his soul, or "Thetan." (Cruise is said to be a level 6 OT.)

As Bromley explains, Scientologists believe Thetans have the capacity to enter any life-form, and chose to enter humans because they have the highest place in nature. But some Thetans lost contact with their divine origins and have become trapped in the human body.

Scientologists are taught that Hubbard, who died in 1986, reached the highest level and the power to shed his body to exist as pure Thetan.

This disconnect between theology and practice brings Scientology closer to mainstream American religions. Some 50 years ago, renowned theologian Paul Tillich noted that American religions were taking a gnostic direction: He found it disturbing that so many of us believed we don't really die (to await a final resurrection), but treat death as if it were no real interruption, believing we simply shed our bodies and live as pure spirit.

"The gnostic understanding of the human situation has been one of the most popular ideas in the West for 2,000 years," said the Rev. J. Gordon Melton, who heads the Institute for the Study of American Religion in Santa Barbara, Calif. "And nowhere more than now."