Molly Worthen teaches history at the University of North Carolina at Chapel Hill; her most recent book is “Apostles of Reason: The Crisis of Authority in American Evangelicalism.”

It’s no secret that Pope Francis’s honeymoon period has given way to deep divisions in his church. Liberals swoon over his tweets declaring that refugees are “our brothers and sisters,” while conservatives rail at his critiques of capitalism, his rhetorical shift away from culture war flashpoints and his efforts to liberalize the church’s teachings on divorce. In March, a Polish priest went so far as to pray for Francis’s death. “I pray for the pope in his wisdom to open his heart to the Holy Spirit,” said Edward Staniek, a priest and theologian in Krakow. “But if he does not, I pray for his quick departure to his father’s house.”

How could anyone wish death on a man who washes the feet of drug addicts and refugees? Ross Douthat, a Roman Catholic and an opinion columnist for the New York Times, has written an erudite and thought-provoking, if ultimately unsatisfying, response to that question. He weaves a gripping account of Vatican politics into a broader history of Catholic intellectual life to explain the civil war within the church. This is not just a conflict between the pope’s liberal fans and conservatives who pine for Benedict XVI, but a contest between two different visions of the church’s relationship to modernity. Douthat believes the stakes are high: “With more popes like Francis, Catholic truth will stand on a knife’s edge.”

In his telling, the Synod of Bishops on the Family, convened in 2014 and 2015, revealed Francis’s plan to alter Catholic teaching in ways even more profound than the reforms of the Second Vatican Council. The pope tried — and failed, at least in the short term — to stage-manage the synod’s outcome (although Douthat admits that it was less rigged than similar gatherings under conservative popes). His main goal was to revise the church’s rule that bans remarried Catholics from receiving the Eucharist (unless a church tribunal issues an annulment, declaring their first marriage invalid).

To liberal Catholics, this prohibition ignores the Gospel’s call for mercy, mistakes the nature of committed second marriages and is grossly out of step with lived reality (vast numbers of remarried Catholics already take Communion, especially in the West). But to conservatives like Douthat, the ban derives from Christ’s unambiguous command: “Whoever divorces his wife and marries another commits adultery against her, and if she herself divorces her husband and marries another, she commits adultery” (Mark 10:11-12).

To disregard Jesus’ teaching, Douthat writes, would be far more radical than allowing the celebration of Mass in the vernacular, embracing religious freedom, permitting Catholics to eat meat on Fridays or any of the other reforms of Vatican II — none of which took such explicit aim at Jesus’ own words. To toss out their traditional interpretation would set the church sliding down a slippery slope toward endorsing same-sex marriage, polygamy and “any stable, entangling commitment that fell outside the norms proposed by Catholic sexual teaching.” Before you can recite a Hail Mary, ancient dogmas will crumble into a “theology of situations.”

It is cold comfort to Douthat that most other conservative Christians, including the Eastern Orthodox Church and most evangelical Protestants, have made their peace with divorce and have not yet collapsed into the state of atheistic nihilism that he fears. The Catholic Church, Douthat says, is the last authority willing to say divorce is bad, the last bastion of the scandal of the cross in a pluralistic, hedonistic culture.

Douthat critiques that culture with provocative historical analogies. He ranges from the earliest church controversies to 17th-century clashes between moralizing Jansenists and accommodating Jesuits, who indulged the sinful lifestyles of French aristocrats and anticipated the “tweeting Jesuit apologists” who vex Douthat with their defenses of Francis.

He is hardly the first critic to accuse the pope and other progressives of ancient heresies, but his accounts of complex episodes in long-ago church history are by turns exasperating and intriguing. He compares Francis’s desire to open the sacrament to remarried Catholics to Arianism — named for Arius, a priest in ancient Alexandria who taught that Jesus is distinct from and subordinate to God the father. This heterodox vision of the Trinity roiled the ancient church for centuries, even after the Council of Nicaea deemed it a heresy in 325.

In its essence, Douthat writes, “the appeal of Arianism was the appeal of a more rationalized Christianity” that demystified the paradoxes of the Trinity. It was an early example of an impulse that appears again and again in Christian history: the desire to reason away the enigmas of orthodoxy, to force Christian doctrine to make sense. He sees that desire in modern progressives’ claim that it is absurd to take Christ’s words about divorce and remarriage literally: How can a committed marriage constitute adultery? And just as Francis has avoided proposing bold revisions to doctrine, but instead has laid subtle groundwork for change in obscure footnotes buried in the pages of papal pronouncements, so too did Arian theologians “revive the heresy through deliberately ambiguous formulations, designed to allow what remained a minority viewpoint among the church’s bishops to appear as a consensus view.”

Historical comparisons that leap centuries, cultures and continents are always full of problems, but they are also deeply interesting. Douthat manages in a slim volume what most doorstop-size, more academic church histories fail to achieve: He brings alive the Catholic “thread that runs backward through time and culture, linking the experiences of believers across two thousand years.” He helps us see that Christians have wrestled repeatedly with the same questions over the past two millennia. How should Christians live in the world but not of it? How can depraved human beings make sense of law laid down by a God who defies comprehension?

Yet the problem with argument by historical analogy is that such comparisons have a way of selectively angling the rearview mirror. There are other, more charitable ways to cast historical light on Pope Francis. One might argue that the Catholic hierarchy’s entwinement with state power and wealth from the 4th century until our own time constitutes one long, largely unrepentant heretical act. It is a rebellion against Jesus’ declaration that his “kingdom is not of this world” (John 18:36), that Christians should refrain from serving both God and mammon (Matthew 6:24) and should render unto Caesar only the things that are Caesar’s (Mark 12:17). The pope’s effort to disentangle his church from the excesses of neoliberalism and nationalist politics is a profound act of resistance against the forces of secularization and worldliness, far braver than his opponents’ defense of traditional marriage.

Here and there, Douthat acknowledges the dark underbelly of the anti-Francis backlash. He fears that the pope’s modest accommodation of secular pluralism has brought out the worst in his conservative enemies and unwittingly empowered “resurgent nationalism and civilizational tensions.” But in his tale of the perils to orthodoxy, he ignores the way in which the church’s compromises with secular democracy and multiculturalism have — at least partly — helped Catholics domesticate the chauvinistic impulses that tend to corrupt all religious ideals.

Indeed, what is really driving Francis’s critics? What about that Polish priest? It wasn’t primarily the debate over Jesus’ teachings on divorce that prompted Staniek’s homiletic death wish but the pope’s call for Catholics to take in Muslim refugees. He was outraged at the Vatican’s challenge to politicians like Jaroslaw Kaczynski, the leader of Poland’s ruling Law and Justice party, who combine Islamophobic prejudice against migrants with showy displays of Catholic piety: Catholicism, Kaczynski declared, is one of “the foundations of our identity, our way of life and of being Polish.”

Xenophobic, nationalist venom — not the pope — is the great threat to the modern Catholic Church. In Christian terms, this poison is not mere heresy. This is heathenism, pagan tribalism, pure and simple. Mr. Douthat, pray for us.

To Change the Church

Pope Francis and the Future of Catholicism

By Ross Douthat

Simon & Schuster. 234 pp. $26.