It seems likely that there will be a female president of the US well before there is a self-professed atheist in the Oval Office. Ted Cruz declared last year that someone who does not begin every day on “his knees” (sic) is not fit to be commander-in-chief. Atheism is controversial, in the US as in many other countries around the world. But both its detractors and its supporters tend to portray lack of faith in a divine power as a possibility or danger available only in modern times.

Those in the Cruz camp often view atheism as a marker and a cause of the degeneration of contemporary society; Cruz’s father famously declared that it is the cause of sexual abuse. The defenders of religion point to the fact that all human cultures throughout history seem to have had religious beliefs and practices, and therefore religion is sometimes said to be an essential feature of human nature.

Those on the other side may celebrate “our” freedom from the superstitions that were rampant before the Enlightenment. Christopher Hitchens argued that “religion comes from the period of human prehistory when nobody – not even the mighty Democritus who concluded that all matter was made of atoms – had the slightest idea what was going on”.

Tim Whitmarsh’s brilliant new book about ancient atheism makes a compelling case that various forms of religious disbelief have been with us for the past two and a half millennia, with greater and lesser degrees of cultural prominence. Atheism has had a distinguished and varied lineage. It seems likely that doubt about religion is just as old as religion itself, although there is no way to prove what people believed or did not believe in cultures that have left us no literary evidence.

Whitmarsh makes the illuminating observation that modern, post-Enlightenment atheism has a particular social function: it draws authority away from the clergy, towards the secular “priests” of science. In the ancient world, the conflict between science and religion did not exist, at least not in these terms. But it does not follow that nobody in antiquity ever questioned the traditional stories about the gods, which were often patently ridiculous.

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Classical scholars may turn to Whitmarsh’s book, as I did, with questions about whether the term “atheism” is really the right one for discussing pre-Judaeo-Christian religious doubts and resistance to religion. It is an academic commonplace to distinguish between the “orthopraxy” of Graeco-Roman religion – the focus on collective rituals, sacrifices and festivals – and the “orthodoxy” of modern monotheistic religions. No ancient Greek or Roman ever recited a Creed. Besides, in classical Greek, the word atheos (“not-god”) is usually used to mean “godless” or “against-the-gods”, rather than a person who does not believe that gods exist. But Whitmarsh builds a case that stories about “battling the gods” are actually ways of articulating doubts about traditional religious teaching. He argues that classicists have gone too far in presenting ancient religion as primarily concerned only with action, not faith. As he rightly notes, this historical claim relies heavily on public sources, such as inscriptions, which may teach us a lot about ritual practices but much less about what individual worshippers thought was true and false. Public documents can only give the “official, ideologically sanctioned versions of events”. For this reason, much of Whitmarsh’s work is a careful teasing out of the literary and philosophical sources, including those that exist only in fragmentary form, as he searches for hints of people in antiquity who questioned the gods’ existence.

The ancient Greeks certainly did not assume that the gods are likable or lovable, and hostility to the gods is a familiar trope in Greek literature. The Homeric poems – which were never treated with the reverence afforded to the holy books of the Islamic or Jewish traditions, but which were by far the best known texts of Graeco-Roman antiquity – depict anthropomorphic gods who are very much of this world, and who interact with humans, even fighting with them on the battlefield. Battling the gods was a common enough trope in the Greek imagination that they had a word for it: theomachia. One might think that stories about gods as threats to humans must imply a strong belief in their existence. But Whitmarsh argues that theomachy stories express “a kind of atheism, through the narrative medium of myth”. One key example is the archaic tale of Salmoneus, who claimed to be Zeus, demanded sacrifices to be offered to himself, and created thunder by dragging kettles around behind his chariot. Whitmarsh suggests that this story raises disturbing questions for believers in the gods: “If gods can be fashioned by mortal imitation, how real can they be?”

Facebook Twitter Pinterest A 19th century Greek vase illustration of Zeus abducting Leda in the form of a swan. Photograph: Stapleton Collection/Corbis

One may still balk at the idea that this is really “atheism” in the modern sense. Even the pre-Socratics of the sixth and fifth centuries BC, who are usually seen as the precursors to modern scientific and philosophical inquiry, cannot all be identified as atheists in any straightforward sense. Xenophanes of Colophon declared that “Africans say that their gods are snub-nosed and black, Thracians that they are blue-eyed and red-haired”: but pointing to the limitations of Greek concepts of anthropomorphic deities is not quite the same as denying the existence of all gods.

In the fifth century BC, we reach more convincing examples of people who can be categorised as atheists in the strong modern sense of the word. Protagoras, an important and influential sophist (“wisdom-teacher”), declared at the start of his book On the Gods, “I cannot know whether they exist”. Whitmarsh shows that this claim was closer to outright atheism than agnosticism, since Protagoras went on to argue that what cannot be perceived does not exist. Another sophist, Prodicus, claimed that “the gods of popular belief do not exist”. In the later Hellenistic period, the third and second centuries BC, the nature and existence or nonexistence of the gods was reconsidered by many schools of philosophy. Stoics identified god with nature and fate. Epicureans had a slippery, perhaps confused notion of gods who were composed of a different, thinner type of matter from all other entities in the universe, and lived “between the worlds”, affecting nothing but dreams and the imagination. Sceptics argued that all beliefs rest on “shaky foundations”, including belief in the gods.

Whitmarsh rightly underlines the central importance of ancient Scepticism in the history of atheism; these philosophers came up with a hugely influential set of arguments against a vast range of religious claims. Carneades, an early leader of the school, used a form of “heap” argument (a thousand grains is a heap; take one away, it’s still a heap; but one grain is not a heap; so when does it cease to be a heap?). His argument relies on the Greek assumption that the gods belong to this world, rather than being a different order of being, and he poses the question: are nymphs and satyrs gods? If not, where do we draw the line? Some have interpreted Carneades – whose work does not survive first-hand – as saying only that traditional religion is questionable, rather than that gods do not exist. But Whitmarsh makes the case that Carneades wanted to prove that belief in the gods is logically impossible; the idea that he was only attacking current (Stoic) beliefs about gods comes from the much later, and highly partial, testimony of Cicero.

Whitmarsh, who is primarily a Greek literature specialist, spends much more time with the Greeks than the Romans, and treats Greek religion more sympathetically. But he has a big and provocative story to sketch once he reaches the world of the emperors. He makes a connection between the disparate, scattered world of the Greek city states and Greek polytheism, which was more a set of local cults than a centralised religious system. With the centralising power of Rome, religion also became more centralised and politicised – as did, Whitmarsh argues, atheism.

Since the Romans tended to equate the success of their own empire with divine providence, scepticism about Rome and scepticism about the gods now went together. (Scepticism about the gods was good, it provided a sphere for political dissent for people who wanted to criticise Rome implicitly.) But the “dream” of being able to “imagine the possibility of a world that had left religions behind” lasted for only two or three hundred years. As the Roman empire became increasingly difficult to govern, there was a strong motive for those in power to find an ideological force that could hold the people under imperial control. Christianity proved particularly useful for this purpose.

The Theodosian code was instituted in the fifth century CE, and represented a vast shift away from the old model of Graeco-Roman polytheism. Now, religion was not an infinitely expandable series of cults dedicated to multiple different divinities; all doubt was heresy, and all beliefs except Nicene Christianity were punishable by death. As Whitmarsh insists, individuals probably continued to have doubts about religion throughout late antiquity and the middle ages; but “they were invisible to dominant society and so have left no trace in the historical record”.

This is an invigorating, urgent book that makes an important contribution to a central contemporary debate. One of its chief virtues is that it underlines the gulf between the Christian and pre-Christian eras, without presenting antiquity as an atheist’s utopia. There were no ancient equivalents of wars using religion as a premise (the crusades or jihad). But doubters were put on trial in antiquity, and some were executed, for instance under the Athenian law against “impiety” – including Socrates, who was accused of “not believing in the gods of the city”.

Whitmarsh also reminds us that disbelief comes in many varieties. Democritus the atomist – who lived in the fifth century BC – seems to have had some kind of belief in the gods: he thought that people see gods in their dreams, although they have no explanatory function for the workings of the material universe. Gods exist, but they are irrelevant for scientific inquiry. Democritus was not an atheist in quite the same way as Hitchens, but he was no fool, and he was certainly not brainwashed by religion.

• Emily Wilson’s Seneca: A Life is out in paperback from Penguin. To order Battling the Gods for £20 (RRP £25) go to bookshop.theguardian.com or call 0330 333 6846. Free UK p&p over £10, online orders only. Phone orders min p&p of £1.99.