More vulgar than vulgate, the late Dennis Potter's clay-footed vision of Christ died on screen but will live again on stage.
Son of Man was Dennis Potter's 12th play for television and the end of his apprenticeship as a dramatist. "It's the first play I am pleased with," Potter told Radio Times at the time. It rehearses the story of Christ from his sojourn in the wilderness, through his spreading fame as the Messiah, to his crucifixion but, significantly, not beyond that point.

Paradoxically, it is not a play that particularly presents religion or is suffused with religiosity. In 1980, 11 years after its first transmission, Potter told Bernard Levin: "I wanted to do it without piety." Much more, it is a public play, a drama about government and military rule, protest and revolution, and so it is very much of its time, the late Sixties.

A A Gill, who reviews television for The Sunday Times badly (by which, of course, I mean unenthusiastically), once hazarded that after a week's thought he could come up with nothing made for television that had had a life beyond the medium. After some 10 minutes' thought, I had a list of around 30 programmes that had been adapted for stage, big screen, book, even musical stage; indeed, for "teleplays" of the Sixties it was a pretty common occurrence.

There is no evidence that Dennis Potter ever intended Son of Man for the stage. A year before its transmission, he had been persuaded to adapt his two plays about the character Nigel Barton into a single production for the Bristol Old Vic. Earlier still, he had received a commission from the National Theatre of which nothing came.

But the impetus for a stage version of Son of Man came from Robin Midgley, then running the old Phoenix Theatre in Leicester. Midgley saw the play off air and "got in touch with him within days because I was so overwhelmed, as all of us were, by it. The Phoenix stage was an open space and didn't need designing. It seemed an appropriate play to give space to. So although its form was for those days very much televisual and episodic, it didn't feel that one was asking too much of the play to take the stage. Dennis was very ill at that time and said he was quite happy about us doing the project but worried that he couldn't give it the time and wasn't well enough to travel to Leicester. So really, it was simply a case of me putting it on the stage."

Like the great majority of Potter's teleplays, Son of Man was made in the studio with several cameras (eight altogether, including two hand- helds to lend a documentary feel to the crowd scenes) and was broadcast in monochrome. It was a huge production, with more than 90 actors and extras, a choir of 25 boys and 17 items of livestock, not to mention two elements specially hazardous in an electronic studio - fire and water.

On the stage, Midgley got by with 19 actors and much doubling. "In those days it seemed possible," he says, "whereas now one would be flummoxed by the scale of the thing. The television play was necessarily a piece of naturalism - to say 'here's this chap in the desert believing that he has this mission and, because he believes it, goes through with it' - and on the stage it seemed to me necessary to do the opposite - to create an atmosphere that wasn't domestic, that had some of the scale of epic about it. We used all the cast in a sort of choric way, which seemed to work juxtaposed to the naturalism: it lifted it in theatre terms to something quite stylised. I suppose walking among the audience as he did was quite startling at that time."

The abiding memory of Midgley's production, which came for a season to the much lamented Roundhouse in London (where its author finally got to see it), is that we all filed out in silence at the end, eschewing the regulation curtain call. The play ends with Christ crucified, a long dialogue- free sequence characterised, in its first director's phrase, by "a real tension about the mechanics".

"At the Phoenix," recalls Midgley, "it was necessary for the audience to walk down towards the stage because the exits were at stage level. On the first night the audience just didn't leave. The doors were open, the house lights up, and everybody just sat there with Frank [Finlay] on the cross. You can't very well come out and say, 'Please go now'. So it took about a quarter of an hour for the auditorium to start to empty.

"That night and every night, people would come up on to the stage and put their hands on his feet and say, 'Thank you very much'. And there were tears. I've never known such a thing. And we never had applause at the end. But Dennis meant it to be incomplete. It was the death of a man. There was no question of a rising again."

The play's first screening as a "Wednesday Play" on 16 April 1969 drew a comparatively modest audience of 4.5 million. The BBC felt it deserved better and gave it an unthinkably early repeat on 4 June. It was last televised in 1987.

The Phoenix production, too, was cautiously approached. Says Midgley, "Before we opened there were no bookings at all. It wasn't that we had a few. We had none. All the cast cared about the play so passionately that they went out delivering leaflets. And we asked all the local priests and vicars to 'please, please, come and see it'. A fair number did and all of them must have preached about it on the Sunday after we opened because on the Monday morning we sold out the whole of the rest of the run. It wasn't from the reviews."

Midgley has a persuasive explanation for the audience's initial reluctance. "I suspect that people of a religious nature think, 'Oh dear, if this is Dennis Potter it's going to be blasphemous. I'm not going to like it', and if they're not religious they think, 'I don't want to come to this because it isn't my territory' and that does make for a problem. You have to get it on and running, which of course you can't do on television but you can in the theatre."

Potter's is a vernacular Christ, more vulgar than vulgate. This sits uncomfortably for some, but there is a strain of demotic poetry that makes this Messiah's direct approach seem human-scale and so part of us. He admires a cross for the craft in it: "It's good timber, this. Hewed with the grain from the heart of the tree. I could fill a room with tables and chairs with wood like this."

His later works, like Pennies from Heaven and Secret Friends, were, Potter argued, far more religiously intended than Son of Man. The latter marks the last expression of Potter's agnosticism. It is indeed a play that affords quite distinct experiences to its audiences, interacting with the beliefs they bring to it. And at its heart is a message that bridges and overarches all religions and none, a fervent plea for tolerance and forgiveness - love your enemy.

n Bill Bryden's new RSC production of Dennis Potter's 'Son of Man', with Joseph Fiennes as Jesus, previews at the Barbican Pit, London EC2, from tonight; opens Tuesday. Booking: 0171-638 8891

n W Stephen Gilbert's 'Fight and Kick and Bite: The Life and Work of Dennis Potter' is published by Hodder & Stoughton on 19 October