"The turnouts have been mainly standing room only," he says. "Some want to know how to refute the book; some want their belief in it strengthened.... Even if people can't wholly accept what Dan Brown has to say, they feel he has touched on something they want to discuss."
Dr. Plumer is now writing a book on why the novel has struck such a chord despite dozens and dozens of books published to debunk its claims.
Those claims include that Jesus married Mary Magdalene and their bloodline still survives; that the idea of Jesus' divinity did not exist until Emperor Constantine formed the Council of Nicea to establish it; and that the Roman Catholic Church has conspired to hide this throughout history, even to the point of murder.
The novel is first and foremost a fantastical murder mystery, an intriguing page turner that grabs even those wholly opposed to its thesis. It catches people's imaginations, many say, because it involves a conspiracy.
"Americans love a conspiracy theory," says Lynn Garrett, religion editor at Publishers Weekly. "It also tapped into people's disillusionment with the Catholic Church following the sexual abuse scandals."
Some say Mr. Brown's controversial approach to history plays on people's limited knowledge.
"One reason it works so well on readers is that he tends to begin with a kernel of something historical and then quickly spins off into fiction - or you could say falsehood, since he represents it as something researched," says Timothy Beal, professor of religion at Case Western Reserve University in Cleveland.
Professor Beal used Brown's novel last fall in a course on the New Testament and early Christianity, illustrating pop culture interest in the topic. "Half of the students had already read the book and many believed it," he says.