The Remigia cave, about eighty miles north of Valencia, features paintings dating from around 6500 BC. Some depict bands of archers hunting ibex; others appear to show executions. These are the ones tourists come for. But the most significant image is the least dramatic. Fourteen individuals gather closely together, watching a lone figure departing from the group. It appears to be an ostracism – a social death, not a physical one.

The hunter-gatherer tribes of that era were perhaps the most equal communities in human history. But this egalitarianism was strictly bounded. Individuals who were not part of the tribe or who broke its norms were cast out or killed. Inclusion required exclusion.

In a famous essay, the economist and philosopher Amartya Sen pointed out that we are all in favour of equality. We just disagree about whether we mean equality of money, or power, or respect, or legal standing, or whatever. The question is ‘equality of what?’ But there is an even deeper question than this: ‘equality of whom?’ Where is the line between those considered as equals and those who are not – between the fourteen and the one?

This is the question animating Equality, a landmark work of intellectual history by Dartmouth historian Darrin McMahon. ‘Time and again we have seen controversies play out over equality’s “substance” and the degree to which it could admit of difference,’ McMahon writes. ‘Did equality imply common religious or national belonging? Was it delimited by sex, title, or race? Or did it free up individuals to make claims on the collective regardless of the fortunes of their birth?’

It is easy to invoke equality without facing its limits. Contra John Lennon, it is actually very hard to imagine a world with no countries. ‘For all the high-minded talk of “global equality” in recent times’, McMahon writes, ‘its contours have most often been imagined from within the walls of nation-states, where equality extends only to those who share a passport and more often than not a place of birth.’

McMahon has set himself an almost impossible task: to analyse humanity’s most powerful and contested idea throughout history and across the globe. Most attempts at total histories of ideas fail. Depth is sacrificed to achieve breadth, the reader is marched along too strict a chronological path or the author gets stuck in an etymological quagmire. But McMahon succeeds. This book is deeply researched, tightly argued and sparklingly written. It ought to be read by anyone interested in equality, and also anyone interested in people, history, God, politics, religion, nationalism, war or love.

The book is structured around what McMahon calls ‘figures’ of equality, a term he uses in the rhetorical sense of a ‘figure of speech’. These figures are explored in roughly chronological order, from ‘Reversal’, the overturning by hunter-gatherers of the dominance of our ape ancestors, all the way to ‘Dream’, the invoking by 20th-century reformers such as Martin Luther King of a new concept of equality founded on universal brotherhood. The only downside of this approach is that it involves a degree of repetition.

There’s no romanticisation in these pages. Not only did hunter-gatherers kill or expel in order to maintain order, they also formed hierarchies. Or rather, hierarchies formed them. McMahon insists that hierarchies are everywhere in human history, just as they exist in every primate community. Human beings ‘cannot live without hierarchies’, he writes, since ‘status is part of the air we breathe’.

One of the big advantages of human hierarchies is their diversity: there’s more than one way to be top dog. McMahon writes that ‘unlike animals, we regularly inhabit multiple hierarchies at once, with the result that a low-status individual in one environment, say a janitor at a corporation, may be a high-status individual, the captain of the company softball team, in another’. This insight is not developed, but it is critical. One way to square equality with hierarchies is to scramble them, not only over generations but also over the course of an average day. In other words, you defang hierarchies not by denying them but by multiplying them.

While hierarchy is a human constant, the term itself is of Christian origin. One of the most important Early Church fathers, Pseudo-Dionysius, coined the term, describing hierarchy as part of God’s ‘perfect arrangement’, especially in the celestial realm – there are, after all, archangels as well as mere angels – and the ecclesiastical one, with archbishops above bishops, and so on.

But at the same time, religion in general (and Christianity in particular) has been among the most propulsive forces for equality in the last two millennia. Both Jews and Christians learn that each of us is made in the image of God. Early Christians lived essentially as communists, while Early Church leaders, following Christ’s example, were outspoken critics of the wealthy. St Basil the Great, for example, told his fourth-century congregation in Caesarea that ‘the more you abound in wealth, the more you lack in love’ and that a rich person who failed to help a poor person was a ‘murderer’.

McMahon explores the role of trinitarian theology as a foundation for equality. The triune Christian God consists of three separate persons (Father, Son and Holy Spirit) who are ‘consubstantial’, sharing the same essence (or substance), and therefore equal. This idea helps Christians to explain how people can be unique, separate individuals and yet made of the same divine stuff and therefore equals. ‘God’s children … likewise share a common nature,’ McMahon writes. ‘The mystery of the Trinity seemed to reaffirm the essential likeness of human beings in relation to one another – seemed to reaffirm their essential equality – however different they might be.’

To modern readers, much of this might seem like the stuff of a Sunday school lesson. But while it is clear that for much of modern history the Church has not been a good advertisement for equality, it is also clear that much modern egalitarian thinking rests on theological foundations. As McMahon writes of 18th-century reform efforts, ‘The very fact that equality was on the horizon at all owed much to these varied Christian efforts. Over the course of centuries, Christians had made of equality a moral good, investing it with a sacral status … Equal was how God had made us; equal was how God intended his beloved to be.’

St Paul famously claimed that the incarnation demolished old distinctions, so that ‘there is neither Jew nor Greek, slave nor free, male nor female; for you are all one in Christ Jesus’. But the question for Christians is the same as the question for all those advocating equality: what does the word mean in the real world, in the thick of daily life, for politics and economics? McMahon notes that religious claims of equality often had little purchase in the material world. St Paul instructed Christians to obey their earthly masters. There was no condemnation of slavery as an institution. Master and slave were equal in God’s eyes. But right here and right now, their position was unchanged.

The Roman Empire in which Christianity flourished had its own abstract ideal of equality under natural law for all male Roman citizens, omnes homines aequales sunt (‘all men are equal’), a principle that became part of the legal code itself under Emperor Justinian. McMahon distils the partnership between secular and religious forces: ‘Thus did the Roman law and Christian theology work together, each in its own way, to situate equality amid inequality, while concealing inequality in equality itself. The one justified the other. And as both the empire of Christianity and the empire of Rome grew, so did that complementary and reinforcing function.’

The question of what makes up the substance of equality occupied the finest theologians for centuries. It is central for secular egalitarians too. And some of the deepest thinkers on this question come not from the Left but from the Right. Perhaps McMahon’s greatest achievement is to take the equality claims of the Right, including those on its extreme end, seriously. The origin of the word ‘fascism’ is fascis, the term for a bundle of rods with a protruding blade. This was an emblem of magisterial power in ancient Rome but also of connection, community and equality. There are fasces flanking the speaker’s rostrum in the US House of Representatives. There is a fascis underneath each of Abraham Lincoln’s hands in his memorial sculpture in Washington, DC.

The fascists of the 20th century were dismissive of liberal versions of equality, in part because these ducked the hard realities that must be faced to achieve equality within a group: first, a clear definition of that group; second, the deliberate exclusion of others. Fascist thinkers were explicit about the exclusionary implications of equality. The Nazi legal scholar Carl Schmitt wrote in 1923 that ‘the question of equality is precisely not one of abstract, logical-arithmetical games. It is about the substance of equality.’ What would this substance be? Schmitt said it could vary. It could be religion, belief, nationhood, tradition or ‘ideas of common race’. But the key concept was ‘equality of type’, or Artgleichheit. The idea of the Volk provided a broad umbrella, shaped by common history, culture, language and experience. But, over time, fascist thinking, especially in Germany, developed a more explicitly racial typology, leading to the genocidal implementation of Schmitt’s argument that, to flourish, societies require ‘the elimination or eradication of heterogeneity’.

The success of fascist politics was down to its clear signalling of who would be the winners, the equals, in a new political order. Fascist scholars and leaders understood that the desire for recognition within a necessarily unequal society created resentment, which could be amplified and weaponised. Drawing on the work of the Dutch scholar Menno ter Braak (who committed suicide in 1940 rather than live under Nazi rule), McMahon offers a chilling but necessary reminder: ‘Where the belief in equality prevailed, resentment would find a place. With the consequence that democratic societies would always produce a steady stream of the very poison that could be used to kill them off.’ The desire to be seen and valued can curdle into reaction and hatred. ‘All human beings seek recognition,’ McMahon writes. ‘And as populist politicians of the Right have arguably understood far better than most in recent years, politics is well placed to provide it.’

The rancour of modern politics is an obstacle to the practical pursuit of greater equality. Right-wing nationalists are dusting off the playbooks of the 1920s and 1930s, whether they admit it (or even know it) or not. In the face of growing concerns over immigration, the proto-fascist French thinker Maurice Barrès wrote at the end of the 19th century, ‘the idea of the fatherland implies an inequality, but to the detriment of foreigners, not, as is the case today, to the detriment of French nationals.’ Meanwhile, too many on the Left are practising a rancorous identity politics of their own, in which, as McMahon writes, ‘white heterosexual men are cast as uncertain allies and privileged exceptions to the rest of humanity’.

There is some hard politics ahead of us, for sure. If we are to stand any chance of cultivating a humane reimagining of equality, we will have to do some hard thinking too.