From left, Lagertha (Katheryn Winnick), Erlendur (Edvin Endre) and Kalf (Ben Robson) in “Vikings.” (Jonathan Hession/HISTORY Copyright 2015)

The clash of religion, politics and conquest in the History Channel’s hit series “Vikings” slowly maps out how the Nordic warriors gave up their gods and adopted Christianity after centuries of warring and raiding.

The tensions ooze amid bloody battles depicting early Scandinavian culture in the first three seasons, which has made “Vikings” a top-rated cable show, with 4.3 million average viewers, multiple Emmy nominations and a fourth season to start Thursday (10 p.m. Eastern).

In the last season, a Christian missionary who shows up in Kattegat, Denmark, is hauled before Queen Aslaug (Alyssa Sutherland) for preaching against “false gods” and put to the test: carry a metal bar that glows red hot to prove his god is stronger than the Norse gods. After he crumbles to the ground screaming in pain with bloody, blistered hands, the Vikings laugh uproariously, and the queen tells her guard to kill the Christian.

In the same episode, Aslaug’s husband, King Ragnar Lothbrok (Travis Fimmel), leads a Viking army in its first siege of medieval Paris. During peace talks with the Parisian military commanders, Lothbrok demands to be baptized so he can see his Christian friend — a priest named Athelstan — in heaven. His Viking captains are shocked when they find him undergoing this conversion.

The drama is loosely based (or largely imagined) on the life of a Nordic farmer-turned-explorer named Ragnar Lothbrok. It’s partly a study in leadership as Lothbrok gains power and must maneuver carefully (and violently) among Viking chieftains and European kings to keep that power.

Wars fill the show with ax and shield battles and gory torture scenes. But, amid that violence, we find plot twists involving fraternal feuds, strong characters, including sword-wielding blonde women with fearsome haircuts, and beautiful cinematography from Ireland of stark fjords, fauna and flora.

Admirers of Oxford philologist J.R.R. Tolkien’s homage to European epics, myths and languages in his classic literary works “The Hobbit” and “The Lord of the Rings” series may find Vikings as greater historical catnip than, say, HBO’s hit fantasy program “Game of Thrones.”

Scholarly works indicate that Lothbrok (also known as Ragnar Hairy Breeches) was, indeed, an early Viking hero described in Old Norse poetry and sagas. An Anglo-Saxon Chronicle says he was the father of many sons including Inwaer (Ivar the Boneless), Halfdan Ragnarsson and Hubba (Ubbe). These sons became Viking leaders after him and avenged his death by leading later invasions of England.

The 12th-century Icelandic poem Krákumál describes Loðbrók’s marriage to a daughter of dragon slayer Sigurd (Siegfried) and shieldmaiden Valkyrie Brynhildr (Brunhild), Germanic and Nordic legends. The show depicts that daughter as Aslaug.

Director Michael Hirst has built his reputation on period pieces in “The Tudors,” “The Borgias” and “Elizabeth.” A student of history who spent time at Oxford and the London School of Economics, he writes the episodes based on his research.

Hirst ponders one of the most fascinating elements of Viking history — how the marauders went from pagans to Christians around the year A.D. 1,000 at the tail end of the Viking age.

“They were formidable warriors, but their culture, their gods, everything else about them was deliberately suppressed by the monks and by the Scandinavian Christians when, of course, after 400 years or so all the Scandinavian countries became Christian, and they pulled down the pagan temples,” Hirst said in an interview with Writer’s Guild of America West in 2013. “They made every effort to destroy any evidence of pagan life and beliefs.”

Some viewers are upset that some Christians portrayed in the series appear cowardly in the face of the violent pagans. But the show portrays — among other things — the civilizing and inhumane practices of both the Norse men and the early English.

The show does not delve deeply into the truth claims of either religion. Rather it focuses on human efforts to practice these religions in a time of civilization clash.

In the first three seasons, much of the conflict centers on a Catholic priest named Athelstan (played by George Blagden), whom Lothbrok takes captive from a monastery in Northumbria. Athelstan adapts outwardly to the culture of his captors (a Stockholm syndrome of sorts) but shows a deep tension of heart and soul as he becomes part of Lothbrok’s family, acting as a translator, warrior in training and armchair sociologist who observes the Vikings with a mix of wonder, fear and witness.

In the final episode of the second season, “The Lord’s Prayer,” Athelstan teaches Lothbrok the Lord’s Prayer before a battle of subterfuge and backstabbing leaves Lothbrok atop the pile of Viking chieftains, a king. The implication is that Lothbrok benefits (or at least doesn’t suffer) by showing openness to the Christian god, violating Viking norms and challenging the Norse orthodoxy.

Certainly, we know England was nearly overtaken by Scandinavian Vikings, except for the resistance of one region called Wessex. That thread in the series, while out of sync with historical timelines, is portrayed strongly. It was the resistance of Alfred the Great of Wessex who inspired Hirst to produce the series in the first place.

We also know that around A.D. 1,000 — perhaps after years of interacting with and capturing monks and women from Christian lands — the men and women occupying Norway, Iceland and the rest of Scandinavia converted to Christianity and, centuries later, made Lutheranism the state denomination. Their people spread throughout Britain, continental Europe and the United States, influencing the English language and culture.

In the show, Athelstan and Lothbrok’s friendship causes jealousy and distrust in Lothbrok’s boat builder, Floki (played by the clever Gustaf Skarsgård). While Athelstan prays one day, Floki sneaks up and bludgeons him to death.

“Martyrdom was, I thought, a really good way for him to die because he embraces it, and it’s the most wonderful Christian thing to become a martyr for his faith and he goes to his martyrdom joyfully,” Hirst told Variety during an interview in April.

Lothbrok grieves Athelstan’s loss and wears his crucifix throughout the siege on Paris in season 3. In the season’s final episode, the French rulers agree to give Lothbrok a Christian burial, when he tells them he is dying and would like to see his friend, Athelstan, in heaven. A group of sturdy warriors hauls his coffin into the Cathedral of Paris, at which point Lothbrok pops out and kills the Catholic archbishop who had tried to deny him a baptism earlier. Lothbrok then lets in the rest of the Vikings to raid Paris.

As the Vikings return to Scandinavia, they leave Lothbrok’s bear-like brother, Rollo (played by Clive Standen), behind with an army to camp near Paris. The French King Charles, Charlemagne’s grandson, has the idea to marry his daughter, the pious Gisla, to Rollo. In season four starting Thursday, we will see that drama begin to play out.

Books such as “A History of the Vikings’ by Gwyn Jones include references to a Viking warrior named Rollo who did invade Paris in the 9th century, may have married a daughter of Charles the Simple named Gisele, swore allegiance to her father in a peace accord, was baptized as he converted to Christianity and ruled what became Normandy. Rollo’s body is buried in the Cathedral of Rouen, and he is said to be the great-great-great grandfather of William the Conqueror.

The show doesn’t present us with perfect Christians or sanitized Vikings. It shows us the messy, bloody, deceitful way of humans in a quest for power and in self-interest. In that process, it also shows us how the idea of Christianity eventually won out over the pagan religions of the North, changing a fierce community of warriors into some of the most civilized and peaceful people on the planet.

Paul Glader is an associate professor and director of the McCandlish Phillips Journalism Institute at The King’s College in New York City. He is also a Swedish/German American. Find him on Twitter @PaulGlader.