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.
For centuries, Mary Magdalene has been seen as a prostitute repenting for her sins. Brown thinks she has been intentionally misrepresented. "Nowhere in the Bible does it say Mary Magdalene was a prostitute," he said.
Furthermore, Brown says that the church -- run by men -- hoped to hide the truth about her relationship with Jesus.
In "The Da Vinci Code," Brown says that Jesus and Mary Magdalene were actually. married. Brown is a novelist, not a religious scholar, but is it possible that some of his claims are correct?
The Rev. Richard McBrien, a Catholic scholar, says there's no factual basis for the belief that Mary Magdalene was a prostitute. "That common belief is simply false," McBrien said.
The Gospels describe Mary Magdalene as someone who had been possessed by devils and had been healed by Jesus.
McBrien said that "we have to appreciate what demons were in those days." Because health problems were so frequently misunderstood, people who were sick were often said to be possessed by "demons."
Unfortunately for Mary Magdalene, she is also introduced in the Bible right after a story about a prostitute whom Jesus forgives for her sins. And in the year 591, the pope at the time -- Gregory the Great -- gave a big Easter sermon declaring that Mary Magdalene and the unnamed prostitute were, in fact, the same person
The Vatican eventually corrected that impression -- but not until 1969 -- 1,378 years later. The damage to Mary Magdalene's reputation was done.
Karen King, author of "The Gospel of Mary," said, "As historians, we can't really say what people's motives were. What we can do is we look at the effects. And the effects were clearly to undermine women's authority in the church."
Many scholars say that real clues in the Bible about Mary Magdalene have been suppressed.
"When Jesus rose from the dead, according to the accounts in the New Testament, in three places he appeared to Mary Magdalene first," McBrien said. "She was the primary witness, even over Peter. So she had all the credentials to be an apostle."
But does any of that mean that "The Da Vinci Code" is right -- that she was Jesus' wife and the mother of his child?
There is little evidence to support Brown's claim, but there are some scholars -- even Catholic ones -- who are willing to entertain that possibility.
"If someone came in and said, 'Look, we have evidence now, it's incontrovertible evidence that Jesus was married. Guess who it was?' I'd say it's only a short putt to Mary Magdalene. If he was married, it's obviously Mary Magdalene," McBrien said.
"Good Morning America" Weekend anchor Bill Weir reported from Temple Church in London, home of the Knights of Templar.
The Temple Church, built in the 12th century, is one of only three surviving examples of round churches in Britain. It is also the home of the Knights of Templar, a sect of warrior monks formed in 1119 that protected the pilgrims going to and from the Holy Land during the Crusades.
The Knights then became the American Express of their day -- holding vast sums of money for kings and popes on both sides of a war.
But were the Knights of Templar also the caretakers of Mary Magdalene, as Brown's book claims? Was she the secret of their order, the very reason they were formed?
Robin Griffith-Jones, the Reverend and Valiant Master of the Temple, says absolutely not. He has written an upcoming book called "Da Vinci Decoded" which debunks the best seller.
On Oct. 13, 1307 -- Friday the 13th -- Philip the Fair, the king of France, arrested the knights on charges of heresy, accusing them of everything from ritualized sodomy to urinating on crosses, said Griffith-Jones. He seized their treasury and kept it all.
The arrests, interrogations, torture and trials took five years and were spectacularly brutal, said Griffith-Jones. The knights were wiped out and buried in The Temple Church -- their secrets buried along with them.
But do those secrets include information on Mary Magdalene? Griffith-Jones says no.
"The real secret of the Knights of Templar has nothing to do with Mary Magdalene and everything to do with international banking," he said.
So does this put an end to the speculation surrounding the mysteries of "The Da Vinci Code?" That's highly unlikely, considering a film version of "The Da Vinci Code" is set to be released in 2006, directed by Ron Howard and starring Tom Hanks.