The danger of atheism was in the breakdown of law and order. I think Thucydides’ record of the plague of Athens in 430–29 and 427–6BC stands for itself.

[2.47] the supplications made at sanctuaries, or appeals to oracles and the like, were all futile, and at last men desisted from them, overcome by the calamity… [2.52] Bodies of dying men lay one upon another, and half-dead people rolled about in the streets and, in their longing for water, near all the fountains. The temples, too, in which they had quartered themselves were full of the corpses of those who had died in them; for the calamity which weighed upon them was so overpowering that men, not knowing what was to become of them, became careless of all law, sacred and profane. And the customs which they had hitherto observed regarding burial were all thrown into confusion, and they buried their dead each one as he could. And many resorted to shameless modes of burial because so many members of their households had already died that they lacked the proper funeral materials. Resorting to other people’s pyres, some, anticipating those who had raised them, would put on their own dead and kindle the fire; others would throw the body they were carrying upon one which was already burning and go away.… [53.4] No fear of gods or law of men restrained; for, on the one hand, seeing that all men were perishing alike, they judged that piety and impiety came to the same thing, and, on the other, no one expected that he would live to be called to account and pay the penalty of his misdeeds. On the contrary, they believed that the penalty already decreed against them, and now hanging over their heads, was a far heavier one, and that before this fell it was only reasonable to get some enjoyment out of life.

Atheism came from disaffection and immorality, in the Greek mind, and it led to the loss of law and order, and divine retribution. It made sense, then to police atheism to some degree at least; especially when the consequences (loss of law and order and divine retribution) could be felt. There are a number of examples in our sources of individuals put on trial for atheism in Athens in the fifth and fourth centuries. These are very controversial. Each example has been undermined for one reason or another — because the source material is unreliable, or the date is strange, or by semantic muddling (as with Socrates’ accusation: by arguing it wasn’t really about atheism). That’s too complex to get into here, but suffice it to say that those supposed to have been prosecuted for atheism are:

Diagoras of Melos. Prosecuted for ‘offence of word rather than action’ according to Lysias: divulging the Mysteries and chopping up a statue of Herakles to use as fire wood. Ar. Birds 1071–87, and later in Lys. 6.17. Athenagoras Plea 4, Suda sv. Diagoras [Suda Online: δ,524]; The Mysteries: Melanthios FGrHist 326 F3, Krateros FGrHist 342 F16. (Likely) Euripides was accused of atheism and prosecuted for it. Arist. Rh. 1416a., also Satyrus, Vita Eur. C10. Herakles as mad: POxy 2400, 3rd Century AD, probably based on Satyrus. (Probably didn’t happen) Anaxagoras. Reputation for natural philosophy. Brought to trial under asebeia (impiety) for ‘not believing in the gods’ (τὰ θεῖα μὴ νομίζοντας, according to Plutarch), ‘because he declared the sun to be a hot ball of metal’ (διότι τὸν ἥλιον μύδρον ἔλεγε διάπυρον, according to Diogenes Laertius) instead of a god, and ‘declared the firmament was made of stones’ (‘Ἀναξαγόραν εἰπεῖν ὡς ὅλος ὁ οὐρανὸς ἐκ λίθων συγκέοιτο’). Ephorus, FrGrHist 70 F196 ap. DS 12.39.2; his fate: DL 2.3.8, 2.3.12–15; Plut. Per. 32.1–3. (Likely) Protagoras. Reputation for natural philosophy. Protagoras’ work Concerning the Gods opened with a statement of what was perceived as agnostic atheism (unbelief, but not disbelief): ‘Concerning the gods, I cannot ascertain whether they do or do not exist’ (περὶ μὲν θεῶν οὐκ ἔχω εἰδέναι οὔθ᾽ ὡς εἰσίν, οὔθ᾽ ὡς οὐκ εἰσίν). Prot. F3 Graham 2010: 696–7; Eusebius, PE. 14.3, 7; Arist. F67 Rose = Sophistes F3 Ross. Arist. ap. DL 9.51–4. He was described by the Old Comic poet Eupolis as ‘an offender in regard to celestial matters’ in 421BC. DL 9.50 = Eupolis F157, a fragment of the 421BC Flatterers. Diogenes of Apollonia. Reputation for natural philosophy. Demetrius of Phalerum FGrHist 228 F42 ap. DL 9.57, Athen. 12.60 = 542e. Damon of Oea. Reputation for natural philosophy and hanging out with other atheists. [Arist.] Ath. Pol. 27.4. Plut. Per. 4.2. Theophrastus. For undermining the gods. DL 5.37, Ael. VH 8.12. Stilpo of Megara. Prosecuted for impiety for arguing that the Athena of Pheidias was not a god, and at the trial defended himself by arguing that he had said she was not a god but a goddess. DL 2.116

Outside of Athens, there is not a great deal of evidence on trials for atheism or impiety. Plato’s Socrates, in the Euthydemus (271b-c), remarks that the sophists Euthydemus and Dionysodorus were exiled from Thurii. Thurii was a Athenian-led colony: it was likely heavily influenced by Athenian culture, not least because it was partly the brain-child of Protagoras and Pericles. In the late 320s or early 310s, before his trial in Athens, Demetrius of Phalerum was exiled from the Libyan polis Cyrene, a Theran colony (Plut. On Exile 601). There is no evidence that Greece in general, then, including Athens before the Peloponnesian war, people were very concerned to prosecute atheistic views as impiety. The heightened sensitivity that resulted in the common accusations in Athens of the fifth and fourth centuries appears to be due to unique circumstances at the time.

I think the key here is that individuals with a reputation for atheism are being used — much in the same way as witches have been — as a scapegoat for the woes of the community. Regardless of the historicity of the trials, it seems clear that trials for atheism were viewed as a plausible consequence of public atheism. There’s also a small amount of evidence of book burning and some other weird things. It seems clear that the deterrents were significant enough that atheists would have been very unwise to air their views in public. That fits well with our evidence: close cabals and sympotic contexts, etc. As Hussey (1995: 536) has observed:

There were in fact some who were atheists, though they had good reason to keep their opinions hidden.

Or Sutton (1981: 37) before him, on Critias:

an obvious question: if these atheistic ideas were Critias’ own, why should he care to advertise the fact? There would be little to gain and much to risk both in terms of personal reputation and of political chances.

Here I’m going to be talking about atheism as Other; as l’étrange: that is, oppositional self-definition of religious identity in Greece. If you want an introduction to the concept of other, I recommend starting with Simone de Beauvoir’s Second Sex and Michel de Certaeu’s Possession at Loudun. Seeing the atheist as other is, I think, the most important ‘use’ of ancient Greek atheism. The importance of the category of l’étrange — the strange; a powerful sense of the uncanny and otherness — has most recently been observed by Whitmarsh: ‘[t]he history of atheism cannot be just that of those who profess not to believe in gods; it must also account for those social forces… that construct it as the other, the inverse of true belief.’ (Whitmarsh 2016: 116)

Even in the Odyssey, atheism began to be conceived as an ‘other’ that was set against, and helped to define, normative religion in Greek society. This raises the question: what is legitimate and illegitimate religion in ancient Greece? There were many forms of extra- or para-religious beliefs and behaviors, including atheism, but also magic, belief in ghosts, curses, and necromancy. It is impossible to define these practices accurately and non-tautologically, as has been recognized in the scholarship on magic: they are practices that broadly shared the practical techniques and conceptual backdrop of religion, but can be grouped in a distinct category as practices that were, in some contexts, subject to prejudice and caricature as ‘illegitimate’ or ‘superstitious’. ‘The scholarly literature’, anthropologists M. and R. Wax observed, ‘contains two principal approaches to the definition and study of magic: an intellectual and a moral’ (Wax and Wax 1963: 495). This was the perspective of James Frazer, one of the founding fathers of anthropology, who argued that magic differed from religion as threats and coercion against the gods, while religion was humble prayer. Yet Greek practitioners of magic not only made curses or spells (epodai), but also prayers (euchai); both aimed to persuade (peithein) the gods, as Plato’s Stranger observed (Pl. Laws 10.909b). These beliefs and behaviors all played a significant part in ordinary Greek life, but they were perceived as differently ‘legitimate’ by different individuals. In other words, the question I posed above — what is legitimate religion? — was a problem for the Greeks too. The best way for our purposes to study these ‘illegitimate’ forms of religion is to concentrate on the attitudes towards those practices, or ‘caricatures’, more than the practices and beliefs themselves (much the same as atheism). Thinking through ‘illegitimate’ religion allowed individuals to construct and reinforce their own understanding of ‘right’ religion in an environment that often lacked more concrete guidance about appropriate ways to behave and think.

Using this lens, the equivalency presented in many of our sources between atheism and fanatical religion or superstition seems quite obvious. This tradition has informed our modern approaches to religion, and can be traced back to the Greeks. Superstition was one of the categories through which the Greeks defined the religious ‘other’. Through the tradition of writing on ‘superstition’, it is possible to explore the way in which both atheism and superstition were paired as symbols of austerity, as extreme and inappropriate attitudes to the gods, and caricatured to some extent as equivalent or allied phenomena. At the opening of his On Superstition(peri tēs deisidaimōnias, lit. about god-fearing), Plutarch distinguishes between atheism and superstition (Plut. On Sup. 164e):

Ignorance and blindness in regard to the gods (theōn) divides itself at the very beginning into two streams, of which one produces in hardened characters, as it were in stubborn soils, atheism (atheotēta), and the other, as in moist soils, produces superstition (deisidaimōnian) in tender characters.

This idea of good religion being the ideal between two extremes can be traced back through Theophrastus to Aristotelean moral philosophy (and beyond — perhaps into Hippocratic texts, Herodotus, Thucydides, and so on). As Aristotle puts it in his Nicomachean Ethics:

…moral virtue is a mean. How so? Namely, that it is a mean between two vices, one of excess and the other of defect; and because it aims at hitting the middle point in feelings and in actions.

Though criticism of ‘superstition’ was key to developing religious identity, those who engaged in such criticism had to navigate the appropriate levels and types of criticism, and avoid a broader skepticism of the supernatural. This skepticism was a distinct category from other forms of atheistic skepticism, and was about exploring and reinforcing one’s own beliefs through opposition.

Scholars have often struggled to perceive how Greeks recognized and practiced a coherent set of beliefs and rituals without a guiding doctrine. Just as the Greeks ‘invented the barbarian’ (see Hall 1989 and Hartog 1988) in order to define their ‘Greekness’ without needing to articulate explicitly or codify what it was to be Greek, they also ‘invented’ the magician, the witch, the oracle-monger, and the atheist to explore and reinforce their religious and communal identity. In his Possession at Loudun (originally published in French in 1970), the French Jesuit historian Michel De Certeau observed the association and correlation between atheism and other ‘superstitious’ beliefs and behaviors. De Certeau found that the increasing identifications of witchcraft in the 16th and 17th centuries coincided with the emergence of public atheism, or at least anxiety about public atheism. De Certeau theorized that witchcraft and atheism, and anxieties about these, arose as a result of (and helped to combat) the doubt and uncertainty that plagued the societies living under the collapsing medieval theology of the period. De Certeau argued that a communal theology under threat must admit doubt, or figures of doubt (like witches or atheists) in to the communal consciousness, in order to be punished or excluded.

This thesis is borne out in the Greek world: atheism, which could be viewed as part of a package of otherness, allowed for the compartmentalization, demonetization, and punishment of figures of doubt and instability. Keeping in mind the constant rivalry of skeptical and theistic ideas in Greek religion, the generation of oppositional otherness in order to create a target for doubt must have been a part of the ordinary religious environment. However, this would, inevitably, come to the fore during crisis periods (as observed already): the Athenians publicly caricatured, demonized, and finally put to death or exiled a number of those they deemed atheists, the most famous of whom was Socrates. The Socrates of Plato makes this argument; that the Athenians made him a bogeyman through the caricatures of Aristophanes and his other critics (in Pl. Ap. 18b, 18d):

many accusers have risen up against me before you, who have been speaking for a long time, many years already, and saying nothing true; and I fear them more than Anytus and the rest, though these also are dangerous; but those others are more dangerous, gentlemen, who gained your belief, since they got hold of most of you in childhood, and accused me without any truth, saying, “There is a certain Socrates, a wise man, a ponderer over the things in the air and one who has investigated the things beneath the earth and who makes the weaker argument the stronger.” […] But the most unreasonable thing of all is this, that it is not even possible to know and speak their names, except when one of them happens to be a writer of comedies.

Socrates’ role as scapegoat for the city was achieved by the larger scale caricaturing of atheism as part of a set of (unfixed) beliefs and practices that fall under the ‘other’. This was part of a crisis of identity as Athens lost faith in the power of its army, democracy, and customs: ‘what it meant to be an Athenian was a focus of concern’ (Eidinow 2016b: 326). Certain individuals, like suspicious women, as Eidinow has argued, or philosophers like Socrates who came to be seen objects of broadly moral panic and religious deviancy, became collective scapegoats for Athens during times of heightened tensions or difficulty (Moral panic: Eidinow 2016b: 10; suspicious women: 312–26). Attacks against these groups demonstrate, as Hugh Bowden (2008: 56) has argued, ‘a polemical attempt to mark out a particular notion of “proper religion”’. Spectres of atheism, impiety, and forms of ‘superstition’ — each with a tangential and difficult relationship to real people and behaviours — were constructed to be an inversion of normal and desirable behaviours through which the Greeks articulated, explored, and attempted to cement their religious identity.