Philip Pullman, author of the trilogy His Dark Materials, is accustomed to upsetting hardline Christians. In an interview last month, he said George W Bush would make a perfect villain for the Milton-inspired vision conjured up in his work. "Bush has this baying certainty and has imposed this fervent zealotry," he argued. "The Christian right in America is the mirror image of the Islamic fundamentalists." In response, a reviewer on the Amazon website snarled: "Do you really need to support this twisted man?" But that was a relatively muted reaction. In England, the Catholic Herald has denounced Pullman's work as "truly the stuff of nightmares . . . worthy of the bonfire", while Peter Hitchens thundered disapproval in an article titled "This is the most dangerous author in Britain".
Yet the National Theatre's staging of His Dark Materials for a second year is remarkable for many more reasons than its exposure of children and adults to a "dangerous" thinker with an appetite for iconoclasm. When the National's director, Nicholas Hytner, first announced he had asked the playwright Nicholas Wright to adapt the books for the theatre, it sounded a high-risk venture. How could Hytner begin to stage this blasphemous metaphysical mystery tour, in which two children - Lyra and Will - negotiate multiple worlds, join forces with armoured bears, consort with witches and fight a whole series of repulsive creatures? How could he excavate the intellectual riches of a text heaving with literary, theological and scientific allusions, which visits the land of the dead, dabbles in quantum physics and re-evaluates original sin? Perhaps most importantly, how could he portray each person's daemon, the animal that reflects an individual's soul and character and, in the case of a child, giddily metamorphoses from one creature to another?
The epic result was a landmark in children's theatre and the most technologically ambitious show the National had ever staged. The daemon problem had been solved by beautiful, ephemeral puppets, while computer graphics helped suggest the shimmering boundaries between multiple worlds. The great drum revolve of the Olivier stage spun up and down to allow the action to shift from a claustrophobic Oxford cloister to the Arctic within seconds. The critics may have had doubts, but the first year's run was almost sold out before the press night, and some people went back to see it eight times. It proved that children, far from having MTV attention spans, can sit still for six hours and beg for more, as long as their imaginations are stimulated.
Yet, as the adaptation returns to the stage in triumph a year on, questions remain about the trilogy's suitability for theatre, not least because of the looming screen version. It seems a foregone conclusion that film will prove, as ever, more glamorous and exciting than theatre, able to conjure up armoured bears and convey the fluidity of daemons with far more va va voom than the grinding technology of the stage. A foregone conclusion, that is, were it not for a significant dissenting voice - that of the author himself.
To understand why Pullman believes passionately in the theatrical adaptation of His Dark Materials gets to the heart of his vision. On one level, the staging makes a pleasing connection with Milton's Paradise Lost - which provided both the trilogy's title and its imaginative fuel - given that four drafts in the library of Trinity College, Cambridge prove Milton originally wanted to write his epic poem as a theatrical tragedy. The Milton scholar John Leonard has declared that "Milton would have forfeited [his] cosmic sweep had he presented the action on a stage". But Pullman, rightly, does not believe that his own cosmic sweep has been forfeited.
Instead, he argues that theatre's limitations are in fact its strengths, because they "leave room for the audience to fill in the gaps". An active relationship grows between theatre audiences and the events in front of them, because the audience members must work with their imaginations to make the play come alive. Obviously there are occasions when, due to the quality of the production or the calibre of the script, the relationship fails. Yet when it does work, "something happens and everything is transformed".
Pullman's remarks about theatre should make sense to any devotee of his books, in which the importance of the invisible and the unquantifiable is clear. Even though he has provided a story that stimulates the visual imagination more than perhaps any other contemporary work, his central concept of Dust - the frequently unseeable substance that represents consciousness and sexual awareness - trumpets the significance of an approach to life that goes far beyond appearances. Each time Lyra seeks the truth on a device called an alethiometer or Will cuts through to another world with his subtle knife, they must achieve a state of mind that draws deeply on their spiritual and imaginative resources.
With an education system dominated by crude government targets and cost-efficiency drives, both Pullman and theatre have an important gift to offer children. The requirement that they use their imaginations to gain access to a more intricate and rewarding vision of life is vital not only for surviving childhood, but for negotiating adulthood. It is a tribute to the intelligent complexity of Pullman's work that, despite its anti-Church stance, even the Archbishop of Canterbury, Rowan Williams, is a fan. He has surmised that the National's adaptation may be "one of those theatrical experiences that justifies the whole enterprise of live theatre in our day". Perhaps you don't even need to see it to believe it.
His Dark Materials is booking at the National Theatre, London SE1 (020 7452 3000) until 2 April