To most readers, Tom Holland will be best known for his outstanding popular histories of the Greco-Persian Wars (“Persian Fire”) and the fall of the Roman Republic (“Rubicon”). In the preface to “Dominion,” he vividly evokes the visceral attraction he once felt toward the apex predators who served as the heroes of his first two books: the austere Spartan warriors of the fifth century B.C. and the ruthless Roman generals of the late Republic. Yet these glamorous human tyrannosaurs, he concludes, lived in a moral universe with which he has nothing in common. “The values of Leonidas,” he writes, “whose people had practised a peculiarly murderous form of eugenics and trained their young to kill uppity Untermenschen by night, were nothing that I recognised as my own; nor were those of Caesar, who was reported to have killed a million Gauls, and enslaved a million more.”

The core argument of “Dominion” is that the teachings of Jesus of Nazareth—whatever we may choose to believe about his metaphysical status—mark a watershed in human sensibilities. All of our contemporary “Western” moral and social norms are the product of this Christian revolution of the mind; the Greeks and Romans are, in ethical terms, as remote from us as giant lizards. “So profound has been the impact of Christianity on the development of Western civilisation,” says Mr. Holland, “that it has come to be hidden from view.” “Dominion” traces this hidden history across the past 2,000 years, showing how many points on our moral compass turn out to have been plotted by Christian modes of thought, whether we recognize it or not.

“Dominion” is an immensely powerful and thought-provoking book. It is hard to think of another that so effectively and readably summarizes the major strands of Christian ethical and political thought across two millennia. Mr. Holland is sometimes defeated by the sheer scale of his subject, and several chapters degenerate into Wikipedia-style summaries of Important Men and their Big Ideas: Irenaeus, Origen, Augustine ; Gregory the Great, Luther, Spinoza. But he is outstanding on the titanic doctrinal battles of the late fourth century, when the newly victorious church struggled to reconcile the social radicalism of the early Christians with the maintenance of a stable world order. Indeed, Mr. Holland persuasively argues that this constant struggle between “establishment” and “radical” ethics is the leitmotif of the history of Christendom more generally. Time and again, from Martin of Tours to John Wesley, Francis of Assisi to Pope Francis, Christian sensibilities have been upturned by reformist thinkers drawing bold new consequences from the great wellspring of Jesus’ teachings.

And herein lies a crucial problem for Mr. Holland’s case. If Christian ideas about wealth, gender, sexuality and power have been in constant flux over the past two millennia, how can we speak of a single, distinctively Christian moral sensibility to which we are the heirs? Here Mr. Holland is, I fear, somewhat evasive. In his introduction, he draws out three examples of Christian “trace elements” in the modern world: “the conviction that the workings of conscience are the surest determinants of good law, or that Church and state exist as distinct entities, or that polygamy is unacceptable.” All well and good: but that hardly constitutes a comprehensive blueprint for Western civilization.

The trouble is that Christian ethics, like Walt Whitman, are large; they contain multitudes. Take, most obviously, the great fissure in post-medieval Christianity, between the reformed and Catholic churches. Is each individual entitled to seek out the truth for herself, by the light of her conscience, or is conformity to church authority and dogma the surest route to salvation? It is hard to imagine a disagreement with more fundamental ethical implications. Did Christian ethics take a disastrous wrong turn in 1517? Or was that when they got back on the right track after a millennium-long detour?

Mr. Holland’s argument about the continuing legacy of Christian sensibilities involves selecting one particular winding strand of Christianity—the one that happens to terminate in our present-day value system—as the “real” one. Mr. Holland postulates a golden thread of Nice Christianity, directly linking Jesus’ teachings with the civil-rights movement, the end of apartheid, #MeToo and so forth. When large numbers of actual Christians between Paul and Pope Francis turn out to have subscribed to Nasty Christianity (butchering Albigensians, incinerating sodomites and suchlike), Mr. Holland blithely comments that “the Christian revolution still had a long way to run.” This argument—that everything Nice in our contemporary world derives from Christian values, and everything Nasty in the actual history of Christendom was just a regrettable diversion from the true Christian path—seems to me to run dangerously close to apologetic.

Consider Christian attitudes to slavery. It is perfectly possible to spin a thread that connects the radical egalitarianism of Jesus’ teachings with the abolitionist movements of the 18th and 19th centuries. It is quite true that the earliest organized opposition to slavery came from within Christian communities, above all the Quakers. For Mr. Holland, the abolitionists’ arguments “self-evidently went with the grain of Christian tradition,” with their opponents reduced to “grop[ing] after obscure verses in the Old Testament.”

But if opposition to slavery is really hard-wired into Christianity, why did nothing resembling an abolitionist movement—or even a coherent intellectual critique of slavery—emerge anywhere in Christendom at any point between the first and 18th century? In late antiquity, as Kyle Harper showed in his extraordinary “Slavery in the Late Roman World” (2011), when the church was faced with the problem of adapting itself to the existing Roman social order, it “fundamentally accepted the practice and ideology of slavery.” In only a single early Christian text— Gregory of Nyssa’s sermon on Ecclesiastes 4:1—do we find anything resembling principled opposition to slavery, and Gregory was concerned with the ethical consequences of mastery for the slave-owner, not with the human rights of the slave. It takes a great deal of special pleading to argue, as Mr. Holland does, that the abolitionists of the Enlightenment were drawing on “a principle that derived from the depths of the Catholic past.”

A second problem with the notion of a specifically Christian sensibility, as Mr. Holland notes in passing, is the difficulty of drawing a hard line between Christian and Islamic moral teachings (to say nothing of Judaism). Muhammad’s God taught him that the steep path is “to free a slave, to feed at a time of hunger an orphaned relative or a poor person in distress, and to be one of those who believe and urge one another to steadfastness and compassion.” The Christian and Islamic ethical systems are not identical, but in each case the levels of variation within each religious tradition (from Greek Orthodoxy to Mormonism, or from Alevism to Salafism) are far greater than the differences between the Christian and Islamic systems as a whole. In both cases, sacred books provide sanction for an immensely broad spectrum of possible behaviors (regarding the correct use of wealth, appropriate gender relations, the ethics of violence), along which later Christian and Islamic societies have shifted unpredictably back and forth over time.

—Mr. Thonemann teaches Greek and Roman history at Wadham College, Oxford.