Both books involved in the case are published by Random House
Michael Baigent told the High Court in London there were "fairly specific" similarities with his 1982 non-fiction book The Holy Blood and the Holy Grail.
But he conceded that there were also many differences.
Mr Baigent and co-author Richard Leigh are suing UK publisher Random House for copyright infringement.
They claim Brown's massive blockbuster "appropriated the architecture" of their work, which explores theories that Jesus married Mary Magdalene, that the couple had a child and that the bloodline survives.
John Baldwin, a lawyer for Random House, read passages from the novel out loud in court on Wednesday to stress the differences from the earlier book.
The Holy Blood and the Holy Grail suggested that a secret order called the Priory of Sion existed to preserve the bloodline founded by Jesus Christ and Mary Magdalene.
When asked by Mr Baldwin if there was any mention in the book about the Priory's oath to keep its true nature hidden, or if there was mention of the Priory protecting the tomb of Mary Magdalene, Mr Baigent said "no".
Judge Peter Smith told Mr Baigent it appeared some theories put forward in The Da Vinci Code were "exactly the opposite" of passages in the earlier book.
But Mr Baigent said differences between the two works were to be expected.
"We were writing historical conjecture, and Mr. Brown was writing a novel," he said.
Mr Baldwin also said many of the ideas in The Holy Blood and the Holy Grail did not feature in Mr Brown's novel, which is a mixture of code-breaking, art history and religion.
The third author of The Holy Blood and the Holy Grail, Henry Lincoln, is not involved in the case.