Jesus. Coming soon to a video shop near you

For his latest TV trick, Russell T Davies will bring the Messiah to Manchester. And he's an aetheist to boot, says Daniel Rosenthal

When a television drama shows the Son of God returning to present-day Manchester in the person of a dim-witted loser suddenly capable of spectacular miracles, you can't help wanting to know if its writer believes. So let's get Russell T Davies' religious inclinations out of the way immediately. "I'm an atheist," says the man responsible for Queer As Folk, Bob and Rose and now The Second Coming, the two-part film that begins on ITV1 tonight. "I'd love to die and find out that I was completely wrong about the non-existence of God, but I don't expect I will. I have strong beliefs in people and their capacity to rely on themselves rather than structured religion, which is a very humanist point of view. But I shy clear of calling myself a humanist because people who do that tend to be wankers."

He laughs. But if this generalised dig at secular belief sounds a little cheap, there's nothing flippant about his treatment of faith in The Second Coming. This is a powerful piece of television that deserves to escape being caught up in the kind of knee-jerk tabloid frenzy that surrounded Martin Scorsese's The Last Temptation of Christ (which The Second Coming echoes, though Davies has never seen it), or the underage gay sex scenes of Queer As Folk. It has already survived careful vetting by ITV's lawyers: the makers, RED Productions, received what Davies calls "a fantastic memo" late last year saying that no blasphemy charge would stand up against the film, in which the Messiah is a video shop worker called Steve Baxter.

Early in Episode One, Steve (played by Christopher Eccleston) establishes his divine credentials with a football stadium stunt aptly described by Davies as "a big, fuck-off special effect" and then gives the world five days in which to come up with a Third Testament or face Judgement Day. As the deadline approaches, demonic civilians and policemen attempt to destroy Steve and corrupt his father and close friends Fiona (Annabelle Apsion), Peter (Ahsen Bhatti) and Judith (the magnificent Lesley Sharp), a trio that represent, says Davies, "blind faith, accepting faith and atheism – although it's not as schematic as that sounds." His screenplay makes knowing allusions to Big Brother, The X-Files ("we've even used the same typeface for our on-screen captions," Davies notes with glee), apocalyptic Hollywood blockbusters like Armageddon and Independence Day, and the good vs evil conflicts of the Omen franchise. The generic thrills and computer-generated effects help director Adrian Shergold sustain The Second Coming's often exhilarating pace, yet Davies says they also serve a higher purpose. "The danger is that if you tell viewers a drama is about religion they will go 'Oh, I'll watch that thing with the car chase on BBC1 instead.' The sci-fi and horror stuff is me unashamedly saying that I want to get and keep a large audience for this – and then they might listen to the more serious stuff along the way." The "serious stuff" means the psychological impact on the characters and, by extension, the audience, of the great "What if?" that inspired Davies.

"Asking myself 'What if Jesus came back to earth?' was thrilling," he says. "What if I wake up tomorrow and I'm completely wrong about God, and my error is staring me in the face? What a mind-fuck that would be. Many Christians aren't ready for that – bishops would run away from it. How would that man feel at realising he had become godly? How would his friends feel? The marvellous thing about my job is that I get to imagine those feelings.

"My biggest decision was saying that Steve is Jesus and the drama absolutely knows that he is from the very beginning. This is not a parable, or a film like Network in which [a messianic newscaster played by] Peter Finch is really just an angry man telling the truth. Once I'd made that choice I found it very hard not just to get inside the Jesus figure's head, but to keep him at the centre of the action. In Network it's really disappointing when the Finch character slips out of the action and the film becomes the story of William Holden [as a TV executive]. I didn't want that to happen in my work."

He says that dramatically, the greatest problem was deciding "whether my Christ was or wasn't omniscient. In the early drafts Steve knew everything that was going to happen, which was very clever but killed all of the drama dead. You can't have a lead character who knows it all. So the breakthrough for me in a script that was rewritten something like 57 times was getting rid of his omniscience." Davies solves the problem by having Steve "download" his grand design into his head, complaining to Judith that it's like trying to cram 50 million megabytes into a pocket calculator. "The downloading concept makes him more human," Davies suggests. "If Steve is as lost as anybody else, it keeps him human and makes it a tell-able story."

His choice of setting, too, is designed to keep the supernatural events grounded in everyday reality, so we're still in Davies' beloved Manchester, as featured in Queer As Folk and Bob and Rose. Only this time round, it's "a very deliberately stripped-down vision of the city. It doesn't have the gloss of those earlier series. This had to be a gritty, grim Manchester in which fantastic things can happen." Davies blends this familiar urban setting and intimate, character-driven narrative with big questions about 21st-century faith and the way we treat the planet, our loved ones and our fellow men. He would love to think The Second Coming might makes some viewers reassess their behaviour, but has "never been a great believer in TV drama's capacity to change people's minds.

"Atheists," he says, "won't be convinced that God exists, nor will Catholics see this and say 'I'm wrong.' But I do think that husbands and wives who've been together for 40 years might watch it and finally tell each other whether or not they believe in God."

'The Second Coming' begins on ITV1 tonight at 9pm and concludes tomorrow