For roughly the same reason that a medieval pope, Leo III, anointed another political bully, Charles the Great, better known as Charlemagne. Put simply, they want a “protector in chief.” Facing a political culture increasingly hostile to their beliefs — and a government riding roughshod over their religious freedoms — evangelicals believe Mr. Trump will be the best guardian of their liberties.

“Trump is a fighter,” Mark Burns, pastor of the Greenville, S.C.-based Christian Television Network, told Fox News. “He is the one to fight for Christianity and for our conservative values we hold dear.”

That’s what Pope Leo believed about Charles, king of the Franks, when he personally crowned him king of the Holy Roman Empire on Christmas Day, A.D. 800. Leo had become so unpopular in Rome that in 799 a band of assassins attacked him during a sacred procession and tried to cut out his eyes and tongue. Leo managed to escape and fled immediately to Charles, known as a defender of the church, and asked him to drive his enemies out of the city.

The king was happy to comply: On Dec. 24, 800, he sent the pope back to Rome with an armed bodyguard, with himself marching “in full martial array” into the city. As Charles promised: “My task, assisted by divine piety, is everywhere to defend the Church of Christ.”

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Thus occurred the spectacle of the leader of the Christian church, on Christmas Day, in the great Basilica of St. Peter, placing a crown on the head of the king — and prostrating himself in a Roman ritual resembling an act of emperor worship.

It didn’t matter that Charles had multiple wives and mistresses. Nor did it trouble the pontiff that he had a reputation for ruthlessness, earned during his wars against the Saxons, a Germanic tribe of pagan worshipers. In 782, in the Massacre of Verden, Charles ordered the execution of 4,500 prisoners, apparently for their refusal to convert to Christianity.

Here was a political leader who knew how to get things done, who could get tough with the church’s enemies, who could protect the empire from barbarian invaders. With the church on his side, he would restore Rome to its ancient glory. Sound familiar?

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Thus evangelical voters joined other South Carolina Republicans in choosing Trump, by an 11 percent margin over his closest competitor, to “make America great again.” According to a recent Bloomberg Poll, they believe Trump is the candidate most likely to “keep their family safe” and “would be most feared by America’s enemies.”

Their own fears are causing them to abandon their principles. In America’s historic struggle to protect religious liberty, evangelicals fought hardest for a Bill of Rights that guaranteed equal justice — not only for themselves, but for unpopular religious minorities. They have been the loudest critics of discrimination based on religious identity.

Yet three-quarters of GOP primary voters in South Carolina — dominated by evangelicals — support Trump’s plan to block all Muslims from entering the United States.

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One of the saddest chapters in the history of Christianity is how the courageous church of the martyrs became — with the help of the state — a fearful and persecuting church. Under Charlemagne, the punishment for refusing to be baptized into the Catholic faith was death. Conversion at the point of the sword became a cultural norm. In a letter to the pope, the new emperor explained his expectations of church and state:

“Our task is externally, with God’s help, to defend with our arms the holy Church of Christ against attacks by the heathen from any side and against devastation by the infidels and, internally, to strengthen the Church by the recognition of the Catholic faith. Your share, Most Holy Father, is to support our army with hands upraised to God, as did Moses in ancient days.”

Many evangelical voters seem ready to support Trump’s militancy, whatever form it takes, with hands upraised to heaven. They say they’re willing to endure their candidate’s “idiosyncrasies” because of his “authenticity.”

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Never mind that he is an authentic egotist, or that he is unabashedly crude, proudly manipulative and emotionally undisciplined. What has happened to the evangelical insistence that presidents be people of prayer, humility and integrity?

Like the medieval church, many American evangelicals expect to benefit from their anointing of Trump. Pope Leo’s constituency was granted access to political power and a privileged social status. They were given the opportunity to put their stamp on the empire’s laws and institutions.

But there was a cost for these privileges. Charlemagne wielded as much influence in church affairs as the pope himself. He appointed and deposed bishops, changed the church liturgy, wrote new rules for monastic life and dispatched agents to dismiss priests who seemed to lack education or piety.

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A government that can shut down a mosque can shut down a church. A president who insults entire categories of human beings with impunity will not hesitate to attack any religious community that dares to criticize him.

After his coronation, Charlemagne declared himself “crowned by God, great and pacific emperor, governing the Roman Empire.” By consecrating a brutal political authority, the Catholic Church eventually gambled away its reputation — its spiritual vitality — for the thin gruel of a richer and more secure earthly kingdom.

In their embrace of Donald Trump, many evangelicals seem ready to do the same.

Joseph Loconte is an associate professor of history at the King’s College in New York City and the author of “A Hobbit, a Wardrobe, and a Great War: How J.R.R. Tolkien and C.S. Lewis Rediscovered Faith, Friendship, and Heroism in the Cataclysm of 1914-1918” (HarperCollins 2015).