Now that every film ever made is available at any hour of the day or night to be delivered to your home in a vial and injected into your cerebral cortex while you sleep, the category of the hard-to-find, persistently-unseen, withdrawn or even banned movie is fast becoming extinct. (And please, don't talk to me about The Human Centipede II [2].) For many years we had A Clockwork Orange to cling to. Your choices, if you wanted to viddy Kubrick's film, were to get a pain in the gulliver watching a ropey VHS pirate copy with German subtitles, or to hop over to Paris to see it in a cinema. You had to actually, you know, try.

Even once A Clockwork Orange became as easily obtainable as an actual orange, we always had Ken Russell's The Devils [3] to dream about, the director's cut tantalisingly beyond reach. Would it ever be commercially available?

Well, it still isn't. But the next best thing - the X-rated cut seen in UK cinemas in 1971 - is out next week on DVD for the first time in a handsome new BFI release [4]. The most widely available version had hitherto been the butchered US cinema edit, so it's good to know that our eyeballs can now be scalded by images no less pure and scandalous than the ones which caused such uproar more than 40 years ago.

The film's story, rooted in historical fact, demands nothing less than a visual and sonic representation of hell. Russell obliges - and then some. The sexual hysteria of an entire convent in 17th-century Loudun, sparked by the repressed fantasies of Sister Jeanne (Vanessa Redgrave), coincides with the efforts of Cardinal Richelieu (Christopher Logue) to raze the fortified city's defences and dissolve its independent governance. The focus of these campaigns, one sexual and the other religio-political, falls coincidentally on a single man: Loudun's hulking, deeply flawed but principled priest Father Grandier (Oliver Reed). As the one persistent obstacle to Richelieu, as well as the unobtainable object of Sister Jeanne's desires, he is doubly vulnerable. The film's demonstration of how church and state do each other's bidding to bring down Grandier, and Loudun itself, is as compelling as it is appalling.

Back in 1971, there was a predictable brouhaha over the movie's sexual explicitness, violence and, most of all, depictions of blasphemy that led to the film itself being wrongly branded blasphemous. Even now the imagery is shocking. What's changed is that most audiences will accept the intentions behind it as entirely honourable. (Though before we congratulate ourselves too heartily on our sophisticated response, we should remember that a rescue attempt is unlikely to be made on behalf of Peter Greenaway's 1992 The Baby of Mâcon, a movie comparable in its conscientious horrors, but without a large and vocal following on its side.)

The chronology of the censorship battles over The Devils are documented in a booklet accompanying the DVD, and in the 2002 documentary, Hell on Earth, presented by Mark Kermode (who has been instrumental in gathering up censored footage presumed destroyed) and included among the disc's plentiful DVD extras. (Check out Kermode here [5] discussing the absence from the current cut of the notorious "Rape of Christ" sequence, which was removed before the picture was ever exhibited publicly.)

That's the furore. What of the film? My immediate reaction, not having seen it before this week, is that it's Russell's strongest work by some distance, bolstered by its ferocious, articulate political passion and rendered with a perverse beauty.

To say it shows no sign of its age would be an understatement. The instincts of Russell and his set designer (then-newcomer Derek Jarman) in plumping for stylised expressionism over historical accuracy have been vindicated. The fresh, zinging white tiles and brickwork of the sets was designed both to reflect a phrase in The Devils of Loudun, Aldous Huxley's book about the same historical events, which described what took place in the city as akin to "a rape in a public toilet", and to simultaneously insulate the picture from historical remoteness. The emphasis on the monochromatic led Russell to describe it as "a black and white film shot in colour"; it's interesting to note how the extensive use of white has protected both The Devils and George Lucas's THX 1138 (also released in 1971) from the ravages of four decades and all the intervening trends and fashions.

As promised by the brash opening sequence, depicting Louis XIII (Graham Armitage) dragged up as Venus in a cabaret floorshow, The Devils is a film about performance. Everyone here is putting on a show of some manner. From the King's pantomime cruelty (gunning down Protestants dressed as blackbirds) to Grandier's priapic, strutting vanity; from Sister Jeanne's writhing ecstasy to the grand and repellent orgies it fosters; from the lip-smacking opportunists performing public humiliations on Sister Jeanne to the rock-star-like Father Barre (Michael Gothard), preening in his Lennon specs; and, finally, to the gruelling public execution at the movie's climax, Russell shows torment and fallibility being exploited for political capital. Peter Maxwell Davies's wailing, abrasive score sounds suitably aggrieved by it all, as though the instruments themselves were being tortured along with Grandier.

"The Devils" [6] is released by the BFI in a 2-disc special edition on 19 March.