American moviegoers have flocked to cinemas to see The Exorcism of Emily Rose, a courtroom drama meets supernatural horror flick that opens in the UK in November. So far the film has grossed £39m in America, making it the most popular small budget film of the year.

It is based on the true story of a girl in Germany who died as a result of an exorcism in 1976. It is a pretty lousy film, with bad acting, stale dialogue and a predictable storyline. But its success in America should not come as no surprise. Questions of faith bubble to the top of the American public debate constantly. The drama in the film's courtroom is even very similar to the one currently unfolding in a real life courtroom in Pennsylvania where science and creationism have been pitted against each other in a battle over what should be taught in public schools.

You wouldn't guess it from the steady stream of ungodliness that is broadcast on TV every day, but America is at odds about the way life and the universe should be understood and represented. The tension is apparent in the millions of dollars in 'indecency' fines that broadcasters are forced to pay for some of their most popular content.

In Emily Rose a young woman is dead, her body emaciated and mutilated, and her priest (Tom Wilkinson) stands accused of causing her death. Her doctors say she is a 'psychotic epileptic' (explaining her visions and dramatic contortions) and died as a result of being taken off medication. Her priest says she was possessed by demons and that he did everything he could to save her. The scary bits happen in flashbacks.

A cute twist is that the prosecutor is Christian, and the defence attorney (Laura Linney) is an ambitious cutthroat atheist with a penchant for martinis, hired by the Archdiocese to make the problem go away quietly. She develops a soft spot for the priest, and when her key witness dies mysteriously, she lets him take the stand and bases her defence on the difficult argument that demons exist.

The film pretends to be balanced but actually insists that demons are real – not just in order to frighten the audience, as would be the case in an ordinary horror movie – but rather to suggest that demons are evidence for the existence of God. This has even upset some religionists. As one review on Catholicmoviereviews.org scolds, director Scott Derrickson has "a rather Calvinistic theological agenda that seeks to scare people into faith rather than address the serious decision of when to exorcise people or when to refer them to psychiatric care".

But overall, the many Christian reviewers in online publications like Hollywood Jesus or Christianity Today Movies are thankful to Derrickson for bringing questions of faith to the big screen.

According to the film's makers this is the first time courtroom drama has been combined with horror in a dual structure. But there is relief all round in the Christian press that this movie is no gratuitous-violence repeat of The Exorcist. Special effects are kept to a minimum, and not a single head spins. Many of the things that happen to the possessed Emily Rose (Jennifer Carpenter) need to be explicable by natural (rather than supernatural) causes. But there is not a shred of Blair Witch reality to it either. Even with its respectable £10m budget, the film belongs most appropriately in the 3am cable rerun genre.

The Wall Street Journal links the film's overnight success to the distributor, Sony's Screen Gems' conscious and aggressive courting of the Christian media. Clint Culpepper, Screen Gems' president and the film's executive producer told the WSJ, "There's a huge faction in this country who I think will respond to the fact that [Emily Rose] made a choice based on her belief and on her faith." Yeah, and look what happened to her.

Later this year, Screen Gems is releasing The Gospel, a film with a black cast about a young singer who 'turns his back on God and his father's church'. We can assume he pays for this sin, as the marketing is squarely aimed at the Christian consumer: the tagline is 'See it! Live it! Spread it!', there is an email form to tell others about the film, and an 'adopt-a-theater' map of America for advance purchase of group tickets.

This kind of grassroots viral marketing is part of what made Mel Gibson's film The Passion of the Christ such a big hit. An interdenominational group of Christian leaders called the Mission America Coalition (MAC) encouraged their constituency to see it and discuss it. In 2004 The Passion earned more than £207m and, according to the Internet Movie Database, 10th place on the list of box office high scores in America. Internationally, it only made it to 68th.

The same form of grassroots Christian marketing is being put behind Disney's December release of the big budget version of CS Lewis' The Chronicles of Narnia: The Lion, the Witch and the Wardrobe.

Christian groups know the power of the media, and are systematic in the promotion of cultural products they feel meet their values. Mastermedia International Inc. is just one non-profit consultancy that has worked with companies like ABC, Fox and Disney to help them tap into the market of 100 million 'evangelical Christians', the prized 'family-friendly market'.

Outside the US, it's easy to gloat about the primitive nature of America's spiritual state of mind. But the success of films like this is less a question of American gullibility and more a matter of major marketing muscle combining with an effective and media savvy evangelical network.n

Solana Larsen is a journalist and editor based in New York