Thirty-five years ago, Norman Stone won a Bafta for Shadowlands, a heart-wrenching drama about the life and love of CS Lewis, starring Joss Ackland and Claire Bloom. He also directed two dramatised documentaries on the Oxford scholar, an intellectual giant and one of the most influential writers of the 20th-century.

Now Stone has returned to Lewis with a new drama called The Most Reluctant Convert. Set before the 1950 publication of The Lion, the Witch and the Wardrobe, the first of The Chronicles of Narnia series, the film focuses on the turmoil of Lewis’s emotional and intellectual journey from atheism to Christianity.

The childhood loss of his mother to cancer and the horror of the trenches in the first world war played a part in shaping his faith. Lewis was to suffer further tragedy in losing his beloved wife, Joy Davidman, to cancer in 1960, which inspired him to write A Grief Observed, his powerful book on mourning and how faith can survive it.

Stone said the moral and aesthetic questions that Lewis raised, and which are explored in the film partly through his own words, are relevant today. “I did not believe God existed, but I was angry at God for not existing”, Lewis observes at one point.

Whether people today have faith or not, it explains why there is now more interest in Lewis than ever before, Stone said, noting that one Narnia fan site alone has 6 million followers: “He’s sold more books this year than last year. I think it’s something to do with everything else that’s going on in the world.”

Stone believes that people can particularly relate to Lewis in the bleak times of Covid-19, just as they did during the Blitz, listening to his famous BBC radio talks: “Those broadcasts were so successful Pub owners were saying ‘quiet everybody, Mr Lewis is on’ … They’d all tune in. He talked in a way that they all understood, explaining his faith in the context of hardship and the war.”

He added: “When, since the Blitz, have people in Britain known people down the street, next door, or their own family die on a regular basis? This horrific situation should stir people’s own thinking and I believe it does. That’s probably why Lewis is selling more books than ever. I think people will go for this film more now than they may have done 10 years ago.”

The Most Reluctant Convert is a screen adaptation of a one-man play first staged in America in 2016 by the actor Max McLean. Seen by more than 100,000 people, it attracted rave reviews. The New Yorker wrote: “McLean achieves something rarely seen on stage or screen: a truthful, richly textured and witty account of religious conversion.”

Stone said: “Lewis looked truth in the eye, no matter what effect that would have. That comes across in his writing and it’s one of the key things that made him popular. At certain times, society says ‘think a little deeper’. When you get something like Covid-19, then we should be thinking a bit more about things, and I think that’s happening.”

Stone, whose previous dramas include The Vision with Dirk Bogarde and Pied Piper with Peter O’Toole, was “blown away” by the script, but realised that it needed to be reworked for the screen. The film, extended to almost 200 performers, was shot in and around Oxford, where Lewis was a tutor in English literature. They filmed in all the places that he would have recognised, including his college, Magdalen, and his home, The Kilns, where he wrote his Narnia books.

Lewis is portrayed in three stages of life, wandering through his own memories as an older man, played by McLean. Nicholas Ralph (James Herriot in the hugely popular remake of All Creatures Great and Small) portrays him in his 20s and Tom Glenister appears as his friend and fellow Oxford scholar JRR Tolkien, one of his intellectual sparring partners.

The film’s producers include Matthew Jenkins, an executive producer on Kenneth Branagh’s forthcoming Death on the Nile, and Ken Denison, whose previous productions include Disney’s The Lion King. They hope for a release this year.

McLean said Lewis went from “vigorous debunker of religion to the most respected Christian writer apologist of the 20th century” and that the questions he explored are complex, a challenge for a drama: “But the words that we use are Lewis’s. We have the rights. That helps because we’re not as smart as he is. One of the great things about playing this role is it’s fun to be this smart for 90 minutes, to be able to say these words, and really embody them.”

• This article was amended on 15 February 2021 to correct the anticipated release date of The Most Reluctant Convert from 2022 to this year.