Director Antoine Fuqua's 2004 movie King Arthur may be "demystified," as it claims, but that doesn't mean it's historical. The film's Lucius Artorius Castus is a half-Roman/half-Celtic soldier on the verge of retirement, but assigned by a bishop named Germanius to rescue a Roman family north of Hadrian's Wall from the dreaded Saxons. It's a kind of suicide mission, but of course Arthur pulls it off, meeting and marrying Guinevere, Pictish warrior princess, in the process.

History-minded critics have savaged the film, noting, for example, that the Britons and Saxons didn't fight as far north as Hadrian's Wall. But students of church history will find an interesting assertion here: Arthur was a Pelagian Christian.

If Arthur really existed, and if the earliest references to him can be trusted, then it does appear that he was a Christian. The 10th-century Annals of Wales claim that at the battle of Badon in A.D. 516, "Arthur carried the cross of our Lord Jesus Christ for three days and three nights on his shoulders, and the Britons were victorious."

By the 11th century, Arthur is a recurring character in hagiographies. He's a scoundrel in the lives of St. Cadoc and St. Padarn, a king seeking a dragonslaying saint in the Life of St. Carannog, and "the king of the whole of Great Britain" by Caradoc's 12th-century Life of St. Gildas. Geoffrey of Monmouth's History of the Kings of Britain (c. 1136) adds many of the now familiar details of Guinevere, Mordred, and the magician Merlin. But none of the Arthurian texts suggest that Arthur was a follower of the 4th-century heretic Pelagius. This was the British monk who taught that humans could morally perfect themselves—making him persona non grata to the likes of Jerome and Augustine of Hippo.

Fuqua's film, however, goes to great lengths to show a "Pelagian medallion," worn by Arthur, and several scenes contain discussion about his allegiance to Pelagianism, and Rome's opposition to it as a heresy. A deleted scene shows Arthur meeting Pelagius. The date is all wrong—Pelagius was born around 354—but the place isn't. Pelagius was probably from Wales, where Arthur probably had his base, if he existed. And to his opponents, Pelagius's homeland and beliefs were inextricably tied together. Jerome, for example, mocked the former law student as a bumpkin "stuffed with the porridge of the Scots."

The corpulent Pelagius was condemned as a heretic in 416, but his ideas lived on. In 429, writes Geoffrey of Monmouth, two bishops were sent "to preach the word of God to the Britons: for their Christian faith had been corrupted by the Pelagian heresy." One of these was Germanus of Auxerre (the film's Germanius?).

Ironically, it is Bishop Germanus, not Arthur, who gives us one of the period's most dramatic and historically credible encounters against the Saxons. As a raiding party approached shortly after Easter, Germanus led a newly baptized army in a bloodless victory by a shout of "Hallelujah!" "The enemy was panic-stricken, fearing not only the neighboring rocks but even the very frame of heaven above them," recounts Bede. "Many of them, flying headlong in their fear, were engulfed by the river."