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Hollywood actor Tom Cruise
Tom Cruise is 50 years old today. Actors and actresses who trade on their looks rarely enjoy passing this particular milestone, and Cruise has plenty of other reasons to feel miserable.

He's stuck in rainy Iceland, filming a thriller, while his third wife, Katie Holmes, files for divorce and sole custody of their six-year-old daughter, Suri. At the weekend, his spokesman asked the media to respect the couple's privacy. The tabloids thought about this for a split second before splashing with "TOM AND KATIE TO DIVORCE - more details pages 2, 3, 4, 5, 10, 20, 21."

Why the hysteria? Cruise is one of Hollywood's most bankable stars, but he's in the twilight of his career as a leading man. There used to be rumours that he's gay, but no one has ever stood them up. If showbusiness reporters wanted to "out" A-list stars, which they don't, then there are more obvious candidates.

But here's another headline from yesterday: CULT STALKS KATIE. The "cult" is Scientology, to which Tom Cruise has belonged since his twenties. The headline comes from the Sun, whose owner, Rupert Murdoch, took to Twitter on Sunday to describe the religion as "something creepy, even evil".

Tom Cruise is not only the world's most famous Scientologist but also one of its most powerful operatives. Some sources claim he's Number Two to David Miscavige, the leader of the organisation and best man at his wedding to Holmes.

Miscavige - handsome, saturnine, ruthless - was responsible for transforming Scientology from the eccentric self-help sect founded by the science fiction author L Ron Hubbard into one of the most powerful social networks in Hollywood. And Cruise was central to that transformation.

For more than 20 years, the star of Top Gun has publicly saluted the Church of Scientology. His wholesome, reporting-for-duty eagerness has persuaded countless fans to try the therapeutic "auditing" courses pioneered in the Fifties by Hubbard.

Reputedly, no religion in the world extracts more money from its followers. (Many, it should be said, insist that it's worth every cent.) Reaching "Clear", a level of enlightenment that strips you of all inherited fears and irrational thinking, could set you back as much as £30,000 over several years. But don't expect to hear about Xenu, the intergalactic dictator who tried to kill billions of souls by stacking hydrogen bombs around volcanoes 75 million years ago. That yarn costs a whole lot more.

In addition to the courses, you need accessories, such as an E-meter, a portable lie-detector with dials and switches that looks as if it was knocked up by BBC special effects for a Pertwee-era episode of Doctor Who.

One way to earn the money for "auditing", as the brain-cleansing technique is known, is to work for one of the Church's military-style organisations. It has built a Disneyfied castle in the grounds of Saint Hill Manor, Hubbard's country house in the West Sussex dormitory town of East Grinstead. There you can see young people strutting around in uniforms that make them look like, well, extras in a Tom Cruise movie.

Tom really was the jewel in Scientology's crown. But the question he ought to be asking himself today, as he contemplates his mid-century, is whether the scales have tipped and he's now a liability.

From a public relations point of view, the Cruise-Holmes divorce is more than a car crash: it's as if Xenu's spaceship (which, according to Hubbard, looked like a DC-8 without engines) had plunged into one of those volcanoes.

Katie Holmes has filed for divorce in New York, where it is thought she stands a better chance of being granted sole custody than in Scientology-saturated California. The message sent out from her supporters is unequivocal - and one calculated to appeal to parents everywhere: she wants to drag little Suri out of the strange world of auditing, E-meters and "Thetans" - floating souls trillions of years old who inhabit human bodies.

But could Scientology harm Suri? Scientology's online enemies have been quick to jump in. Tony Ortega, editor-in-chief of New York's Village Voice and a ferocious opponent of the Church, reckons that "what may have convinced Katie to run was the frightening prospect that faces all Scientology kids beginning at six years old - a form of interrogation known as 'sec [security] checking' ".

Ortega has posted on the Village Voice website a list of questions drawn up by Hubbard entitled, "Children's Security Check Ages 6-12". It begins: "What has somebody told you not to tell? Have you ever decided you did not like some member of your family? Have you ever gotten yourself dirty on purpose?"

There's no evidence that Suri was going to be "sec checked". A spokesman for the Church said yesterday that he'd never come across Hubbard's document except in material from the group's critics (though he didn't deny its authenticity). But ex-members of the group who grew up in it claim to have been subjected to inquisitions when still very young - the aim, they say, being to extract information about their parents.

Scientology often has to fend off accusations of spying on members and former members. There were reports yesterday that sinister men in sunglasses were stalking the New York apartment where Katie Holmes has taken Suri. Nothing to do with us, said the Church.

It's also claimed by critics that Tom Cruise himself is closely monitored by his Scientology minders. Again, it's denied - but one can understand why Miscavige might regard his star turn as a dangerously loose cannon these days.

Remember the time he jumped up and down on Oprah Winfrey's sofa to declare his love for Katie? Scientologists squirmed. The Church also threw a fit when the gossip website Gawker hosted an "indoctrination video" - intended for internal consumption only - in which Cruise spoke passionately but incoherently about the meaning of his religion. (Gawker's founder, Nick Denton, has flatly refused to take it down.)

"When you're a Scientologist, and you drive by an accident, you know you have to do something about it, because you know you're the only one who can really help," Cruise said.

Cruise also features prominently in the biggest single PR disaster to hit the Church of Scientology since Hubbard unveiled his "spiritual healing technology" in 1950. This was an episode of South Park entitled "Trapped in the Closet" in which the character of Tom hides in Stan's closet and flatly refuses to come out.

Far more damaging than the references to rumours about Cruise, however, was the episode's animated Xenu, based on Hubbard's own esoteric teachings and captioned, "This is what Scientologists actually believe." Which it is. Or, to be more precise, it is a secret imparted only to the tiny minority of Scientologists who have handed over truckloads of money to reach Operating Thetan level III.

But that doesn't mean ordinary Scientologists don't know about it. As recently as a decade ago, if you mentioned the top-level teachings to a church official, you would probably cause massive offence: such knowledge was so potent, hinted Hubbard, that it might even kill the person who misused it.

In an age of Gawker, YouTube and hundreds of anti-cult blogs, however, all of Hubbard's revelations - plus allegations of serious financial mismanagement by rogue Scientologists - are available to any would-be recruit with access to the internet.

Scientology's own websites have also been hacked into by the internet-based activist group Anonymous, members of which have staged demonstrations against the Church's alleged policy of "disconnecting" members from their families. This loosely organised but savage mockery of Scientology drew furious responses from its lawyers - but to little effect.

It's worth noting, though, that Anonymous only started its cyberwarfare in response to the Church's clumsy attempts to suppress the Tom Cruise video in which the star gushed so embarrassingly about the efficacy of Scientology at the scene of road accidents.

David Miscavige's policy of cultivating Hollywood must have seemed inspired when posters of Cruise hung on the walls of millions of teenage girls. But that was before the actor embarked on a policy of marrying progressively younger women (the last two Mrs Cruises were each born 11 years after the previous one, meaning that number four should be 23) and saying things that confirmed suspicions that he isn't the smartest Thetan in the volcano.

Put it this way. Xenu was once the most closely guarded secret in the Church of Scientology. Today it yields 451,000 results on Google - thanks, in part, to the film star Miscavige once described as the world's most dedicated Scientologist. And now it looks as if Rupert Murdoch is using the divorce as an excuse to declare war on the organisation. Watch your back, Tom.