Early Christian history dominated the second day yesterday of the hearing in the High Court into whether the American author Dan Brown had infringed copyright by lifting ideas from a book published more than 20 years previously for his blockbusting fiction, The Da Vinci Code, written in 2003.

Michael Baigent and Richard Leigh are suing their publishers, Random House, for breach of copyright, saying that Brown took at least 15 core ideas from their non-fiction work, The Holy Blood and The Holy Grail, written in 1982. It was the turn of the defence yesterday to deny the claims and, as ever, the devil was in the detail.

Both books touch on the hypothesis that Jesus married and had children, and that his descendants are alive today. But John Baldwin, QC, for the defence, told the court yesterday that the two books approached the thesis differently, and often used different parts of the legend.

Copyright did not protect ideas, it protected only the expression and treatment of ideas, Mr Baldwin said. And many of the ideas common to the books, now known in court as DVC and HBHG, had been around for a long time and were to be found in many other sources.

Many ideas in HBHG were either ignored or interpreted differently in DVC. He gave the example of the Knights Templar, set up in the 12th century as the military and administrative arm of the Priory of Sion to protect pilgrims on their way to Jerusalem, according to HBHG. Sixty years later, in 1188, the two organisations fell out.

At this stage Mr Justice Peter Smith interjected from the bench. “It’s not surprising, given what happened in 1187.” The court was briefly silenced. “The loss of the kingdom of Jerusalem, thanks to their stupid master,” the judge explained. Knowing nods all round.

“That’s not in the DVC, that’s for sure,” Mr Baldwin said, regaining the initiative.

The judge replied: “Mr Brown is writing a novel. The last thing people want to read in a novel is a long historical treatise.”

Mr Baldwin, seeking to prove that DVC puts a different interpretation on well-known facts from HBHG, said that, in DVC, the Knights Templar were founded to find the documents that proved Jesus had issue and that his bloodline had entered the early Merovingian kings of medieval Europe.

The claimants argue that there are at least 15 instances where Brown directly plagiarised their work. The defence denies all 15, saying that although there may be similarities there are distinct differences of detail in each case. Mr Baldwin also argued yesterday that, although there were similar themes, HBHG covered a much wider canvas than DVC.

“Many of the ideas complained of are not even in both books, and some are not even in either,” Mr Baldwin said.

He said that the complaint was that HBHG disclosed the conjecture that Jesus was married to Mary Magdalene, that they had children who married into a line of French kings, and that there was a secret society based in France dedicated to restoring the line to several European thrones. That, he said, was not part of the plot of DVC.

The theory that Jesus was married had first been aired in an article in The Observer in 1971, he said. “We say the claim relies on and seeks to monopolise ideas at such a high level of generality they are not protected by copyright.”

Mr Baldwin, whose case largely relies on showing that HBHG covers a wider range of topics than DVC, and that many of its ideas are in the public domain, drew two parallels. You cannot, he said, claim copyright over a few bars of a song; it’s the whole tune or nothing. And a novelist who plans a chase on a mountainside and looks up a table to find the height of the mountain cannot be accused of breaching the copyright of the man who measured the mountain.

Besides, Mr Baldwin claimed, Brown had written the synopsis for DVC, containing most of its core ideas, in the laundry room of his parents’ house long before he had consulted HBHG.

The judge, who said that he had read both books, adjourned the case for the rest of the week to read them again. The hearing resumes on Tuesday.

Explore Books

• Book Extracts

• Book Reviews

• Books Group

• Audio Books

Times Recommends

• Superfreakonomics by Steven D Levitt and Stephen J Dubner
• The Times picks the 50 best paperbacks of 2009
• Vincent van Gogh’s letters show the soul of the artist