The veteran picked to lead the Defense Department has praised the brutal religious military campaigns of the past and has called for a Christian approach to governing.

Before Donald J. Trump picked him to lead the Department of Defense, Pete Hegseth spoke often about a medieval military campaign that he saw as a model for today: the Crusades, in which Christian warriors from Western Europe embarked on ruthless missions to wrest control of Jerusalem and other areas under Muslim rule.

As he embraced a combative brand of Christianity in recent years, he wrote that people who enjoy the benefits of Western civilization should “thank a Crusader.” On his arm, he has a tattoo with the words “Deus Vult,” which he has described as a “battle cry” of the Crusades.

“Voting is a weapon, but it’s not enough,” he wrote in a book, “American Crusade,” published in May 2020. “We don’t want to fight, but, like our fellow Christians one thousand years ago, we must.”

Mr. Trump has so far stuck by Mr. Hegseth as his nominee for secretary of defense despite a growing series of disclosures about his past, including allegations of sexual impropriety, alcohol abuse and financial mismanagement. Mr. Hegseth has vigorously denied the allegations, calling them an attempt to disrupt Mr. Trump’s agenda.

The issue of Mr. Hegseth’s religious expressions has come up in the past. He has said he was barred from participating in the military security detail for President Biden’s inauguration in 2021 because of a tattoo on his chest depicting a Jerusalem Cross, a religious emblem that was also a symbol used by crusaders. (Reuters and others reported that his tattoos, including the Deus Vult motto that has been used by white supremacists, prompted a fellow service member to flag Mr. Hegseth as a potential “insider threat.”)

Mr. Hegseth’s interest in the Crusades — campaigns that featured so many atrocities that many Christian leaders today view them as a shameful stain on the religion’s history — is linked both to his Christian beliefs and worldview. In his books, he says that if the United States cannot mount a successful defense against Islamist and leftist ideology, the nation will be destroyed and “human freedom will be finished.”

The Crusades are a frequent touchstone in contemporary American debates about Christian power, said Matthew Gabriele, a professor of medieval studies at Virginia Tech. When Barack Obama, then the president, referred to the period in a speech at the National Prayer Breakfast in 2015 as a time when people “committed terrible deeds in the name of Christ,” some conservatives took his remarks as an affront.

The far right has also embraced crusader iconography and language with increasing openness, including at the Unite the Right rally, a white supremacist gathering, in Charlottesville, Va., in August 2017 and at the riot at the U.S. Capitol on Jan. 6, 2021.

A lawyer for Mr. Hegseth, Timothy C. Parlatore, said that crusader symbols are common among military veterans like Mr. Hegseth, who served at Guantánamo Bay in Cuba and in Iraq and Afghanistan. He said that the display of crusader symbols by some white supremacists did not mean that Mr. Hegseth intended that meaning.

Mr. Hegseth grew up in a Christian home. Even so, he told a Christian magazine in Nashville last year that he underwent a religious transformation in 2018, in his late 30s, when he and his current wife, Jennifer Hegseth, began attending Colts Neck Community Church in New Jersey. The church’s pastor, Chris Durkin, remains close to Mr. Hegseth and recently recorded a video supporting his nomination, describing him as “a genuine patriot who loves God.”

Mr. Hegseth and his family moved to the Nashville area about two years ago, a decision he has said was based on their desire to send their children to Jonathan Edwards Classical Academy, a Christian school founded in 2009. Classical education, in which classrooms focus primarily on the Western canon, has become a fast-growing movement among conservatives who are wary of secular public schools.

In Tennessee, the Hegseth family joined Pilgrim Hill Reformed Fellowship, a small church opened in 2021 as part of the growing Communion of Reformed Evangelical Churches. The denomination was co-founded by Doug Wilson, a pastor based in Moscow, Idaho; his religious empire now includes a college, a classical school network, a publishing house, a podcast network and multiple churches, among other entities.

Mr. Wilson is a self-described Christian nationalist, which he defined in an interview with The Times as someone who sees that “secular nationalism doesn’t work,” and who wants to limit the power of the government to impose restrictions on Christians.

He has written prolifically for years, but his profile within conservatism has risen in the Trump era; he was interviewed on Tucker Carlson’s podcast this year and was invited to speak at the National Conservatism Conference. The Communion of Reformed Evangelical Churches has expanded recently to include about 150 congregations, including Mr. Hegseth’s in Tennessee.

In his writings, Mr. Wilson has argued that slavery “produced in the South a genuine affection between the races,” that homosexuality should be a crime and that the 19th amendment guaranteeing women the right to vote was a mistake. He has written that women should not ordinarily hold political office because “the Bible does say that when feminine leadership is common, it should be reckoned not as a blessing but as a curse.”

The governing documents of the Communion of Reformed Evangelical Churches include a statement that women should not “be mustered for combat.” (Mr. Hegseth has also said that he does not believe women should serve in combat roles.) The church reserves leadership positions for men and asserts that men are the heads of their households, views shared by many theologically conservative churches.

In an interview on Wednesday, Mr. Wilson said he had never met or communicated with Mr. Hegseth but expressed enthusiasm about the prospect of his leadership of the Defense Department.

“I would hope Pete Hegseth would be a disrupter of the Pentagon’s way of doing things,” Mr. Wilson said. “I would like to see a rebuilt military that’s far more lethal and a lot smaller.”

Mr. Hegseth told the Christian magazine in Nashville that he was studying a book by Mr. Wilson; on a podcast Mr. Hegseth said that he would not send his children to Harvard but would send them to Mr. Wilson’s college in Idaho.

Mr. Wilson is among the Christian leaders who in recent years have reframed the Crusades, which included mass killings of Jews and Muslims, in a positive light. In the interview, he described the campaigns as imperfect, even horrifying at times, but also as “a long overdue reaction to Muslim aggression.”

Mr. Hegseth has written that while the Crusades were filled with injustice and unspeakable tragedy, the alternative would have been “horrific,” because it is Western civilization that has nurtured the values of “freedom” and “equal justice.” His writings warn of the growing presence of the Muslim faith in the West, and urge Americans to work on issues such as education, media and law to protect Christian values.

“We’re in middle Phase 1 right now, which is effectively a tactical retreat where you regroup, consolidate and reorganize,” Mr. Hegseth said on a 2023 podcast affiliated with Mr. Wilson’s church. “And as you do so, you build your army underground with the opportunity later on of taking offensive operations in an overt way. And obviously all of this is metaphorical and all that good stuff,” he added before breaking into laughter.

In his book, Mr. Hegseth also offered a nod to the prospect of future violence: “Our American Crusade is not about literal swords, and our fight is not with guns. Yet.”