People often ask, "How much of The Da Vinci Code is true?" I wearily answer that Paris is in France, London is in England, and Leonardo da Vinci painted pictures. Let's look at four areas where Dan Brown's history is bunk.
Constantine the Great
First is his treatment of Constantine the Great (d. 337), first Christian emperor of Rome. After adopting the Christian God as thanks for a dramatic military victory in 312, Constantine initially seems to have imagined him as the "the Highest Divinity" among other gods. But he swiftly gave tolerance, favor, and subsidies to Christians, even before becoming sole Roman emperor in 324.
These moves were not, as Dan Brown claims, an effort to curry favor with the "new Vatican power base." Not only were Christians just a small and unimportant minority of the population, the pope wouldn't reside at the Vatican for another thousand years. As historian A.H.M. Jones says, Constantine "would not on any rational calculation of his interest have chosen to profess Christianity."
The emperor called bishops to the Council of Nicea in 325 to settle disputes among Christians, not between Christians and pagans. He provided free transportation, served as honorary chairman, and proposed homoousion, "of the same essence" to describe the relationship between the Son and the Father. He didn't invent the Divinity of Christ, choose the canonical Gospels, nor rewrite the New Testament.
Constantine did continue to control all pagan cults through his imperial office of pontifex maximus--a title still held by the pope. Yet he forced no conversions and didn't devise a hybrid religion as Dan Brown claims. Neither was baptism forced upon him while dying. The emperor had delayed his baptism, as many adults did in those days. He'd had been planning an elaborate ceremony when he fell mortally ill. The Eastern Churches honor him as a saint, "the Thirteenth Apostle."
Then Dan Brown grossly distorts the history of the Knights Templar by making them secret goddess-worshippers destroyed by the cruel Catholic Church. The real Templars were an order of warrior monks founded in 1118 to protect pilgrims in the Holy Land. This wasn't, as Brown absurdly alleges, a cover to excavate documents about Jesus and Mary Magdalene on Temple Mount.
Far from being masters of esoteric mysticism, the Knights were largely an illiterate bunch. They didn't invent Gothic architecture and their several round churches aren't proclamations of crypto-paganism.
The Templars lost their reason for existence in 1291 when Muslims drove the Crusaders from the Holy Land. But they retained great wealth which King Philip IV of France coveted. Having persuaded himself that the Knights were depraved heretics, he had all members in France arrested in 1307. After horrible tortures, a hundred of them confessed to idolatry and immorality, crimes that persuaded the pope to suppress the order in 1312. Scores of Templars, including their last Grand Master, were burnt at the stake. None of this happened in Rome as Dan Brown describes because the pope of the time, who was dominated by the French king, resided in Avignon.
Priory of Sion
Dan Brown piles on a further distortion by making the Knights mere puppets of the Priory of Sion, a secret organization that since medieval times has protected the holy bloodline descended from Jesus and Mary Magdalene. Many notable figures from politics, science, and the arts including Leonardo da Vinci supposedly belonged. For a rare original touch, Brown also makes them worshippers of the divine feminine through sexual rites.
But the Priory of Sion was a hoax perpetrated by a convicted French conman named Pierre Plantard. In 1956, he and several friends had established a short-lived club of that title named for a hill in France, not Temple Mount. Plantard and his confederates later prepared phony genealogies and other documents "proving" his descent from Christ via the Merovingian dynasty of France. His great secret was supposed to shake the foundations of Christendom and make him king of France. These claims became of basis of Holy Blood, Holy Grail by Michael Baigent, Richard Leigh, and Henry Lincoln (1982) but were definitively debunked by BBC in 1996. Plantard was forced to admit his fabrication to a French court and died in 2000. So the con game was over before Dan Brown commenced his novel, but somehow he failed to notice this.
Leonardo da Vinci
Brown's wildest distortions cluster around the life and work of Leonardo da Vinci. Exploiting the artist's enigmatic image was crucial to the project. Brown makes Leonardo a "flamboyant homosexual" with occult tastes whose contempt for Christianity inspired him to hide hostile coded messages in his paintings.
Leonardo may have experienced same-sex attraction but he can scarcely be called flamboyant about it. He despised occultism, including the mystic pretensions of alchemy. There was much to criticize in the Renaissance Church but he died and was buried with full Catholic rites.
Brown is flatly wrong on Leonardo's output (seventeen acknowledged paintings, not an "enormous" number and not all religious), his patronage (secular as well as clerical, one not "hundreds of lucrative Vatican commissions"), the medium and location of the Last Supper (it was painted directly on a monastery's dining room wall; it isn't a fresco in the monastery's church), and even the size of the Madonna of the Rocks (six and a half rather than five feet high).
Brown's allegations about the paintings' content are just as wrong. That's St. John, not Mary Magdalene, to the right of Jesus in the Last Supper. He's painted as an effeminate young man according to the taste of the time. Leonardo's St. John the Baptist is--unconventionally--just as womanish. The Eucharistic aspect of the scene is expressed by Christ's hands near bread and a wineglass. No central "Grail" is needed.
The Mona Lisa is traditionally identified as a portrait of a Florentine lady. Perhaps it's just Leonardo's idea of feminine beauty. There's no reason to imagine it as an androgynous portrait of the self-pleasuring artist. The Louvre's Madonna of the Rocks and the National Gallery's copy the Virgin of the Rocks feature the Infant Jesus blessing baby St. John, an unusual but hardly heterodox subject.
Considering how Dan Brown makes hash of historical facts, who would trust his historical speculations?