Ben-Hur, the remake of the iconic 1959 Charlton Heston blockbuster, is due out in the UK this week. The film's US reception hasn't been universally positive and it has so far taken less than it cost to make, but the story of a 1st-century Jewish nobleman's encounter with Jesus brings faith to a mainstream audience – and that has to be a good thing.

But what's sometimes forgotten is that the story itself derives from an 1880 book by Lew Wallace, a Union general, lawyer and diplomat. It was the best-selling American novel of the 19th century and made Wallace very rich.

On the other hand, it hasn't weathered the years well. While the story itself – Judah Ben-Hur's childhood friendship with the Roman Messalla, his betrayal and enslavement, his eventual release and encounter with Christ – remain as gripping as ever, Wallace's Victorian prose is stodgy and the book lacks pace for modern readers.

And that's the verdict of his great-great-granddaughter Carol Wallace, herself a highly successful author one of whose books, To Marry An English Lord, was cited by Sir Julian Fellowes as an inspiration for Downton Abbey.

Carol Wallace has taken the old-fashioned prose of the original and transformed the book into a more accessible and readable format. It's the official tie-in novel to the 2016 Ben-Hur and contains eight pages of colour images from the film. There's also an illustrated hardback collectors' edition featuring more than 100 illustrations alongside film trivia and details of the history of Ben-Hur as a novel and in film.

Asked why she wanted to update the story, Wallace admits: "I never actually read the original version of Ben-Hur, though I was always very proud of my great-great-grandfather. And I thought it was very cool to have such an important author in my family tree. When my father died in 2013, I was prompted to pick up the book. It was clear to me that there was a wonderful story buried in some stodgy nineteenth-century prose. And I thought it would be huge fun to liberate it."

However, she says her famous ancestor had "done all the hard work".

"I knew from the start that all I wanted to change was the mechanics of the storytelling – the tale itself is very strong," she says. "It wouldn't have been adapted so often if this weren't a tale people wanted to experience. I decided that I would not change any of the fundamentals and also that I would not second-guess Lew's research. He did a very thorough job on that for the 1870s, and I saw no benefit in trying to bring more contemporary attitudes or information into the story."

She cut down on the descriptive passages, which Lew Wallace included because the landscape of the Middle East was far less familiar in his day.

"Any contemporary reader knows what a palm tree and a Roman helmet look like, so I could cut a great deal there. On the other hand, I did spend more time on the women's experiences, relating more scenes from their points of view and making them (I hope) more well-rounded. And finally, I wanted this version of Ben-Hur to read more like a thriller, so I did my best to keep the tension up with shorter sentences, short chapters, and as much narrative tension as possible."

One of the things she enjoyed most, she says, was writing about Christ: "My process as a writer is to live, imaginatively, the scenes I write about. And Lew's book – subtitled 'A Tale of the Christ' – begins with the Nativity and ends with the Crucifixion. Jesus is an important character in Ben-Hur and Lew wrote reverently about him, giving him only dialogue from the Gospels. It was deeply moving for me to adapt Lew's narration of these scenes and to project myself into them, which is the only way I know of to really describe a scene or write dialogue. It was, as you can imagine, an amazingly rich spiritual experience."

It's her hope that readers will rediscover not just the power of Ben-Hur, but "the heartfelt faith that Lew himself gained in his writing of this world-famous saga".

Ben-Hur, by Carol Wallace, is published in the UK by Lion Fiction, price £8.99.