Of all the things Pope Francis might have been expected to mention in Fratelli Tutti: On Fraternity and Social Friendship, his most recent encyclical letter, humanism was probably not one of them.

Encyclical letters are the heavy artillery of papal communications, reaching further than the multitude of other letters, speeches, and messages that emanate from the Vatican. In this one, the leader of the world’s one billion or so Catholics wrote about the damage done by ‘illusory’ communication; about the destruction of human dignity in the treatment of refugees; and about the toxicity of a wasteful culture. Among this, in paragraph 86, at the end of a long chapter about the Good Samaritan, he slipped in a mention of ‘humanism.’

This was not an antagonistic reference, despite the association of humanism with secularism and hostility to organised religion. Indeed, the Pope was more critical of (some) religious people than he was of the irreligious: ‘Those who claim to be unbelievers can sometimes put God’s will into practice better than believers.’ Pope Francis seemed to find in humanism precisely what we need to resist the kinds of violent nationalism, xenophobia and contempt for difference that plague the world today.

His former Anglican counterpart in the UK, Rowan Williams, makes a similar point in his forthcoming book, Looking East in Winter. Drawing on centuries of Eastern Orthodox spiritual reflection, the book turns on the idea of ‘liturgical humanism.’ The former Archbishop of Canterbury laments the ‘the loss of an authentic humanism’ in our time, and seeks instead ‘the [kind of] ‘humanism’ in which [there is] a vision of every human face as the focus of self-forgetting love.’ It is in embracing this ‘integral humanism’ – a phrase we will return to – that humanity can face the ‘notably wintry world’ of today.

That such prominent Christian figures speak warmly of a movement that is, in the popular mind at least, basically anti-religious, may seem odd. Why are the Pope and the former archbishop of Canterbury writing so positively about humanism?

The answer lies in its history, which is more complex and interesting than its current formulation suggests. The political philosopher John Gray wrote a book about liberalism having two faces. Humanism is much the same: it has a religious face and an anti-religious one, but the former predates the latter by a long time.

If people are in any way familiar with the idea of religious – specifically Christian – humanism, it will be through eminent Renaissance men of letters, most famously Desiderius Erasmus. The late medieval discipline of studia humanitatis was the study of texts from the ancient world, primarily for their grammatical and rhetorical qualities, and the scholars who practised it were called umanista. Although they often delicately negotiated the different worlds of classical Rome and medieval Christendom, there was little sense that the two were irreconcilable. Indeed, in the words of church historian Diarmaid MacCulloch, ‘the vast majority of humanists were patently sincere Christians who wished to apply their enthusiasm to the exploration and proclamation of their faith.’ This applied not only to Catholics like Erasmus but to Protestants, including John Calvin, whose first published work was a commentary on Seneca the Younger’s De Clementia. Writing and thinking well – humanely – was prized across the theological spectrum.

In time, the term ‘humanist’ drifted away from someone who was essentially a grammarian or rhetorician to meaning someone who studied human nature and human affairs. Thus, when the first publication to be titled The Humanist was published in 1757, the editor, an Irish clergyman, wrote that ‘the title…implies neither more nor less, than that it interests itself in all the concerns of human nature,’ and claimed the magazine was ‘calculated to convey some little useful and entertaining knowledge of various kinds, historical, classical, natural, moral, and now-and-then a little religion into the readers’ minds.’

By the later nineteenth century, the Renaissance had come to be identified as a distinct historical period allegedly characterised by a new approach to human dignity and a clean intellectual break with the Middle Ages. In reality, historians have shown there was far more continuity between the two periods than originally thought. Human dignity was not invented in fifteenth-century Florence. Nevertheless, the image of Renaissance umanista uncovering such dignity for the first time has persisted, and their original interest in style and rhetoric has faded from view. ‘Humanism’ became more about human dignity than prose style. Even after this shift, however, there was no sense that humanism was incompatible with Christianity.

Protestants – or at least some of them – had a harder job reconciling the humanistic commitment to dignity with their own theological understanding of human nature, which could be unremittingly bleak. Saved by faith alone, humans were wretched and helpless creatures, their good deeds little better than ‘filthy rags,’ some argued. Such an uncompromising understanding of human nature faded over time, however, and the early years of the twentieth century saw Protestant publications such as the Humanism and the Bible series.

It was in the Catholic Church, however, that humanism was developed with the greatest commitment and sophistication. The single most important humanist work of the twentieth century came from Catholic Thomist philosopher Jacques Maritain in 1936. It was entitled Humanisme integral – hence Rowan William’s phrase above. Translated into English two years later, the book argued the dominant ideologies of the day – liberalism on one side and socialism and communism on the other – were in danger of flattening or crushing the complex, multifaceted ‘human.’ They risked turning people either into atomistic individuals or abolishing their uniqueness altogether in, and for the sake of, the collective good. In their place, Maritain argued, humanism needed to maintain both human uniqueness and dignity and our indissoluble connection to the wider public good. Humans were, he recognised, ‘individuals’, characterised by their differences from other humans, but also ‘persons’, beings formed by the bonds of love with others.

Along with philosophers such as Étienne Gilson, Maritain’s work was to prove enormously influential. His ideas informed those of his friend Pope Pius XII, whose rhetoric around human dignity and rights in the 1940s helped changed the intellectual climate, making it more amenable to the idea of legally-recognised, generic ‘human’ rights. Maritain’s ideas also helped form the basis of personalism, a movement grounded on a fundamental commitment to the dignity of the human person. Personalism would, in turn, prove hugely influential: shaping the tradition of post-war Christian Democracy in Germany, Netherlands, Spain and elsewhere; shaping the founding principles of the European Economic Community through the auspices of politicians such as Alcide De Gasperi, Konrad Adenauer, Robert Schuman, and Jean Monnet; and finally, in the drafting of the UN Declaration of Human Rights, which mentioned the individual only once but the ‘[human] person’ five times and ‘personality’ three times.

The tradition went on to influence post-war Catholic thought with one Pope after another praising humanism. Pope Paul VI talked about ‘the principles of a true humanism.’ John Paul II wrote about ‘authentic humanism.’ Pope Francis spoke of ‘a genuine and profound humanism’ and now, most recently, of the humanism inspired by faith. In this way, humanism has shown itself to be a thoroughly religious, and specifically Catholic, idea with a long history and  wide reach.

So much for the first face. Humanism is also an anti-religious movement and this constitutes its second face.

If you went looking for an atheist in the nineteenth century you would most likely find them under the labels of freethinker, infidel, rationalist, sceptic, secularist, or unbeliever. Few, if any, called themselves humanists. Nevertheless, as early as 1812, Samuel Taylor Coleridge could write of he ‘who has passed from orthodoxy to the loosest Arminianism, and thence to Arianism, and thence to direct humanism [before falling] off into the hopeless abyss of atheism.’ Coleridge’s humanism was not so much anti-religious as deistic or Unitarian – someone who believed in God but without any doctrinal trappings, as his publisher Joseph Cottle suggested in 1837. In this regard, the commitment to the ‘human’ within humanism was always at least latently non-religious: easily hijacked to mean a commitment to the human as opposed to the divine.

This was precisely how it came to be deployed by the French philosopher Auguste Comte, grandfather of sociology, in the early nineteenth century, as he attempted to put the study of the human on a scientific footing. He set up a religion of humanity, which treated mankind, rather than God, as the object of worship. Comte’s religion borrowed heavily from Catholicism but liberated itself from its more incredible theological and metaphysical claims. Even adapted, it did not flourish, especially in Britain where its elaborate rituals put off a still predominantly Protestant population. The biologist Thomas Huxley once witheringly called it ‘Catholicism minus Christianity.’ It did, however, mutate by the end of the century into a gentler form of ‘ethical humanism’, which emerged in the ethical movement of the late Victorian period.

This comprised a series of societies for people who had lost their (usually Anglican) faith but who wanted to preserve the moral, ritual, and community dimensions of their abandoned religion. The object of faith now, after Comte, was ‘man’ not God. Thus, when The Humanist – not the eighteenth-century journal of that name but a new organ of the ethical movement published in the 1920s – launched its opening edition, its front page editorial was on ‘The Religion of Humanism’ which, it declared, was based on ‘faith in man.’

In this way, the word was finally weaponised against its religious associations. F.J. Gould, a co-founder of the Rationalist Press Association, which was anti-religious, wrote in 1900 that humanism was ‘a new idea [that] is actually displacing Theism… [and which] lies in contrast with Theism.’ Humanism became a commitment to the human as opposed to the divine.

For a time, the two faces coexisted in relative harmony. When, in 1944, the BBC broadcast three talks about humanism it engaged with scientific humanism with Julian Huxley, classical humanism with Gilbert Murray and Christian Humanism with J.H. Oldham. The three did not agree on everything – Oldham, for example, insisted that humanism needed theism – but there was little sense that, awkward bedfellows though they might be, divorce was imminent.

In the post-war period, however, Protestant (as opposed to Catholic) Christians often preferred to see only the non-religious dimension in humanism, and non-religious humanists increasingly stressed their antipathy to religion and, in some cases, steered humanism in a consciously anti-religious direction.

One recent history of post-war humanism shows that in some historically Protestant countries, non-religious humanism was primarily interested in questions of freedom of thought and expression. In these places – such as Britain, where nonconformists had long since done the heavy anti-clerical lifting, and America, where the First Amendment prevented the confessionalisation of the state – humanism didn’t need to be weaponised against religion. In other countries, however, particularly historically Catholic ones, the movement became a battering ram against the still-powerful clerical establishment, and humanist associations and unions founded in the fifties and sixties campaigned on an anti-Catholic ticket, focusing on moral issues around abortion, marriage, divorce, and the like.

This is a helpful categorisation, although it only goes so far. In the UK, for example, Catholicism was much less of a cultural force and the established churches were often in lockstep over cultural liberalisation. Yet that has not stopped anti-religious humanism from growing into a powerful movement. This has much to do with some supremely articulate anti-religious humanist campaigners,  but the rise of anti-religious humanism in the UK is still a subject that requires academic  attention. After all, it is a long time since England or Scotland has had the equivalent of ultramontane Catholicism or American-style religious fundamentalism looming over its public debates. The troubles of Northern Ireland can only explain so much.

However complex its history however, non-religious humanism is now a powerful force in our intellectual landscape, all but eclipsing the longer and richer tradition of Christian humanism. Moreover, unveiling the two faces of humanism helps us navigate the current situation, when for the first time in living memory, there are genuinely anti-humanist threats on the horizon.

What you consider to be anti-humanist does, of course, depend on what you consider to be humanist. But if you judge humanism to involve a commitment to human dignity, rights, rationality, morality and freedom – a list of attributes on which most religious and non-religious humanists would agree – it is not so far-fetched to see how they might be in danger.

In the immediate term, a number of these beliefs will be stress-tested by the rise of populist movements and of authoritarian regimes round the world, whose commitment to a humanistic understanding of the person is dubious at best. However, such threats pale into insignificance when placed alongside the potential danger to humans posed by genetic engineering and AI.

Kazuo Ishiguro’s brilliant new novel Klara and the Sun captures, with characteristic subtlety and narrative intelligence, a world in which humans have been formed and deformed, lifted and left behind by advanced genetic engineering and new AI technology. In a similar way, Martin Rees, Astronomer Royal, recently remarked to me in an interview for Reading our Times, my books and ideas podcast, ‘we’ve got no guarantee that the [artificial] entities who are our progeny a few hundred years from now, will have anything more than an algorithmic understanding of our emotions and how we behaved.’ Novelists and others have been warning for years about the prospect of a ‘Posthuman Future’, to quote a Francis Fukuyama title. Their predictions may now have some substance.

You do not need to be alarmist about a fully post-human future to imagine that our longstanding commitment to the human may be under threat. And it is for this reason that we would benefit from healing the schism in humanism. There are differences between the two faces, as there are within any party, such as in politics. But more important is the shared commitment to the human, which will become vital in the twenty-first century as we are faced with opportunities to edit or remake humanity altogether.