Dan Brown's international best-selling thriller "The Da Vinci Code" stirred up controversy when it called into question fundamental principles of Christianity. In his novel, Brown sets forth the theory that Mary Magdalene was not a prostitute, but instead was married to Jesus and bore his child, and that Jesus' bloodline still exists today.
Brown also claims that the Catholic Church knows and suppresses these stunning revelations, and the truth is guarded by a secret society of visionaries, including the artist Leonardo Da Vinci.
Some critics, such as Daryl Bock, charge that the historical claims in the book are completely false. Bock wrote "Breaking the Da Vinci Code."
"It's the filling in of a blank for which we have absolutely no evidence," Bock said. "I can suggest all kinds of things happened historically if there's no evidence for them and say, hell, believe this. But there is not one shred of evidence."
Even Brown said he began as a skeptic, but his research convinced him and he became a "believer." And the book has struck a deep chord with readers -- 25 million of whom have bought the book.
In a quest to find the answers to these mysterious questions and more, "Good Morning America" sent its correspondents to key locations in the novel to uncover the truth behind the secret society that Brown claims has been guarding this centuries-old secret.
ABC News' David Wright reported from Milan, Italy, from the Santa Maria delle Grazie, home of DaVinci's masterpiece "The Last Supper."
No one disputes that Da Vinci was the ultimate Renaissance man -- a skilled artist and scientist, and one of the most important inventors of his day.
But Brown would add that Da Vinci was a religious radical too, and grand master of a secret society suppressed for years by the Roman Catholic church.
Brown says the proof lies here in a monastery in Milan, where he says the secret has been hiding in plain sight for 500 years -- encoded in the details of the painting.
In "The Last Supper," Jesus reveals that one of his disciples will betray him. Brown says that the figure to Jesus' right in the painting, who has been identified as St. John, is not a man at all -- but is actually Mary Magdalene.
The figure is effeminate, and there's even the hint of red hair, often associated with Mary Magdalene.
Art historian Marina Wallace examined the painting and weighed in with her verdict. "It does look like a woman, you are right," Wallace said. "But it's not. It looks like an angel. A pure, young figure. A sweet, soft, faithful disciple."
John was the youngest disciple, said to be Jesus' favorite, and he is often depicted without a beard.
"You can read all sorts of things into paintings, especially when you don't look at enough paintings," Wallace said. "I don't think there is a secret code in the painting except the wonders and mysteries of Leonardo's mind."
"Good Morning America" anchor Robin Roberts reported from the Church of San Sulpice in Paris. In Brown's book, he claims the church is dedicated to Mary Magdalene and holds many clues about her relationship with Jesus. Church historians say that the church is a shrine to the Virgin Mary.