Although many Greek philosophers preferred naturalistic explanations to explain the workings of the universe, they often shied away from atheism—at least in public. In fact, there is no definitive evidence of self-proclaimed atheists in Greece and the word “atheos” was generally used as an insult. Yet, a significant number of thinkers critiqued religious traditions, made fun of its inconsistencies, denied the existence of the Olympian gods, and even more shockingly, played with the idea of doing away with the gods altogether.
Atheism, and even critique of religion, was rare in the ancient world. People generally accepted the existence of the gods—even the gods recognized by other cultures. We see this especially strongly among the Hellenistic Greeks (the Greeks after Alexander the Great) and later the Romans. Governing over a huge territory, they came into close contact with many other cultures. Comparing their pantheons, they came to believe they all worshiped the same gods, but by different names. For instance, the Assyrian goddess Mylitta became associated with the Greek goddess Aphrodite, the Greek goddess Demeter with the Egyptian goddess Isis, and the Greek god Hermes with the Egyptian god Thoth. This process, known as syncretism, was also deliberately used to increase cohesion among the various peoples under their rule.
If we look really hard, however, we can infer some atheists must have existed. For instance, a psalm in the Old Testament states: “The fool says in his heart, ‘There is no God.’” A hymn in the Indian Rig Veda has a poet similarly denounce those who deny the god Indra:
He about whom they ask “Where is he?” or they say of him […] “he does not exist.” […] Believe in him! He, my people, is Indra. 
Greece became the ideal breeding ground for atheistic thought, as the pre-Socratic philosophers had devised naturalistic theories of the universe ever since the 6th century (as we have read in the chapter “Sacred Geometry”). Yet even in Greece, where atheistic arguments are well-documented, we have no definite evidence of even a single self-proclaimed atheist. This is likely because those accused of atheism were seen as a danger to society and often had to fear for their lives.
Critique of religion does go way back in Greece. The earliest critic might have been Xenophanes of Colophon (c. 570-475 BC), who openly criticized the popular religion of his day. He denied the existence of prophecy and called mythological stories “the fictions of our predecessors.” Most famously, he ridiculed the fact that most “mortals believe gods are born, have clothing, a voice and a body just like them.” He then added:
If horses or oxen […] had hands and could draw with their hands, […] horses would draw the figures of the gods as similar to horses, and oxen as similar to oxen.
The Ethiopians say their gods are snub-nosed and black; the Thracians that theirs are blue-eyed and red-haired. 
Instead, Xenophanes believed in a more abstract god, “one god, greatest among gods and mortals, not at all like mortals in body or thought.” He claimed this god could not be known and required no rituals.
Starting in the 5th century BC, the name “atheos” became attached to the names of various individuals, likely as an insult. This happened, for instance, to Diagoras of Melos or “Diagoras the Atheist”. He was often called “the first atheist,” yet we are not sure whether he rejected the existence of all gods or just the Olympian gods. All we know about his beliefs is based on later gossip. It was said he had lost his belief in the gods when a poet plagiarized his work, swore an oath that he had not done so, but was not punished by the gods. Many witty stories became associated with him. According to one story, he once chopped up a wooden statue of Heracles to roast some lentils, which he called “Herakles’s 13th labor.” When he was caught in a storm at sea, the crew blamed his atheist thoughts for the bad weather, after which he pointed to another ship in trouble, asking: “Do they have a Diagoras too?” He became most notorious for revealing and then mocking the Eleusinian Mysteries, a secretive initiation rite surrounding the cult of the goddess Demeter. Little is known about what happened during these rituals, besides that people were taken into a dark room where they experienced panic, ritually enacting Demeter losing her daughter Persephone, who was taken by Hades to the underworld. Afterward, the initiates were brought into the light, where they supposedly witnessed the goddess Demeter herself and achieved a state of ecstasy. The goal of the whole ordeal was catharsis, a cleansing of the soul, which guaranteed divine protection and a blessed existence after death. Remarkably, even the usually skeptic philosopher Cicero was enthusiastic about his visit. The Athenians usually were quite tolerant about religion, but not about mocking the Eleusinian Mysteries. Diagoras was accused of impiety (asebeia) after which he fled Athens in fear of his life. We are even told an inscription was put up in bronze offering a reward to anyone who either caught or killed him.
According to his critics, Theodorus of Cyrene (5th century BC), went even further. He wrote a now-lost book called On the Gods, in which, he supposedly “did away with all opinions respecting the gods.” Hippo of Samos (5th century BC) was another early atheist. Aristotle complained that he could not see a role for anything in the world but matter. The Sophist Protagoras (c. 490–420 BC) in his work On the Gods took the agnostic position. Unfortunately, only the first line of this work has come down to us. In it, he unapologetically acknowledged the option that gods might not exist. We read:
Concerning the gods, I cannot know whether they exist or whether they do not, or what form they have, because of the obscurity of the subject, and the brevity of human life. 
The treatise caused such an uproar that Protagoras was expelled from Athens and his books were burned.
Protagoras was part of a group of intellectuals connected to the famous general and statesman Perikles (c. 495–429 BC). In Plato’s dialogue called Protagoras, he recounted how his teacher Socrates was excited to be invited to a gathering in which Protagoras was also present. Among the guests were Perikles’s son Xanthippus, the physician Eryximachus, the tragic poet Agathon, and the Sophists Hippias of Elis and Prodicus of Ceos. Prodicus, who according to Socrates had fallen asleep on a pile of sheepskin, was also often accused of atheism. He had supposedly claimed that “the gods of popular belief do not exist.” In his work called The Seasons, he even hypothesized that the gods were nothing more than deified inventors. The god Dionysus, he claimed, originally was the inventor of wine and Demeter of bread. Another influential guest was the general and statesman Alcibiades (c. 450–404 BC), who was later charged with impiety for parodying the Eleusinian Mysteries at parties. Critias (c. 460–403 BC), Plato’s uncle, was also present. At that time, he was a young poet, but he would later become the leader of the Thirty Tyrants who ruled Athens after the city was defeated by Sparta.
Finally, there was the renowned doctor Hippocrates of Cos (c. 460–370 BC), the father of medicine, who, together with Socrates, faced difficulty convincing the doorman to let them enter. Hippocrates favored naturalistic explanations for illnesses. Medicine, Hippocrates claimed, should not be hijacked by “magicians, purifiers, conjurers, and self-promoters, who pretend to be pious and to have some special insight.” For example, in his work On the Sacred Disease, he made the case that epilepsy is not caused by possession, but can be explained by natural factors:
It appears to me to be in no way more divine or sacred than other diseases; it has a natural cause, from which it originates, like other illnesses. People consider its nature and its cause as divine out of ignorance and wonder. 
Although Hippocrates did believe in gods, he claimed they were simply “not responsible” for epilepsy. In fact, he considered it slightly offensive to blame these erratic seizures on the gods.
Although his secular way of thinking was impressive, his theory of medicine wasn’t much to look at from a modern standpoint. He claimed that the body contains four humors—blood, phlegm, black bile, and yellow bile—which he associated with four organs of the body, with the four elements, with four planets, and so on. Disease, he claimed, is caused when these fluids are out of balance. Yet despite the superficial nature of these ideas, he paved the way for secular medicine. His work was later expanded by Claudius Galenus, better known as Galen (c. 129–216 AD), who ventured into anatomy, giving detailed descriptions of the skeleton, the circulatory system, the brain, nerves, and the heart. Galen started out as a surgeon for gladiators and worked his way up to become the medical advisor to various Roman emperors, including Marcus Aurelius. Since dissection of human corpses was forbidden by Roman law, he used primates and other mammals instead.
Secularization also took place in the writing of history. The first historian was Herodotus (c. 480–420 BC). In his major work, Histories, he blamed the war between Greece and Persia not on the gods, but on the actions of a Persian king. However, Herodotus still found a place for divine intervention in his explanations of many historical events. This was not the case for his successor Thucydides (c. 460–400 BC), who was a student of the pre-Socratic philosopher Anaxagoras and was “whispered to be an atheist.”  In his Histories of the Peloponnesian War, he recorded the war between Greece and Sparta, of which he had first-hand experience as an Athenian general. Unlike Herodotus, he left no room for divine intervention in his description of events. All decisions, in his view, were solely based on human decisions.
Critique of religion reached a larger audience when it was picked up by the famous playwrights, who often used their characters to experiment with controversial ideas. The playwrights of Athens wrote tragedies and comedies which were performed at an Athenian festival in honor of the god Dionysus, after which a judge would award a prize to the best performance.
A great example of a tragedy is the trilogy called Oresteia, by Aeschylus (c. 525–455 BC). It fabricated an origin story for the Athenian law courts. The courts in Athens were designed as a secular institution. Religion played no part in its procedures, it was only rarely mentioned in legal speeches, and defendants who shifted the blame to the gods were often mocked.
Now, let’s dive into the story. The Oresteia builds on an older myth about a curse surrounding the family of Agamemnon, which was caught in a cycle of revenge killings. The curse started when Agamemnon’s grandfather Tantalus, tried to trick the gods by serving them his own son Pelops for dinner. The gods brought Pelops back to life, and when he grew up, he prolonged the curse by killing someone. His two sons, Atreus and Thyestes, each claimed the right to be king. Out of anger, Atreus served his brother many of his own children in a stew. One of the sons who survived was Aegisthus, who then killed Atreus. Atreus’s son Agamemnon took revenge and pushed Aegisthus from the throne. When Agamemnon was about to sail to Troy, the winds died down, making him unable to take off. To solve this problem, Agamemnon sacrificed his daughter Iphigenia. The first part of the play starts with Agamemnon returning from Troy. Shortly after returning home, he was killed by his wife Clytemnestra, with the help of her new lover Aegisthus, to avenge the death of her daughter. In the second part of the tragedy, Agamemnon’s son Orestes takes revenge by first killing Aegisthus, and then his own mother, after which he is chased off stage by her avenging spirit. Part three of the trilogy starts with a priestess running from the temple of Apollo at Delphi in fear, claiming that the Furies, deities who take vengeance on human beings for their misdeeds, were after Orestes. When Apollo tried to stop them, the Furies claimed it was their right to take revenge. Then the goddess Athena took over to settle the dispute once and for all by founding a law court on the hill of the Areopagus in Athens. A trial was held between Orestes and the Furies, with Athena as judge, and the citizens of Athens as jury. Orestes emerged victorious, after which the Furies threatened to destroy Athens. But then Athena intervened once more, offering the Furies a home in Athens, not as Furies but as Eumenides (“Kindly Ones”). They accepted the offer, allowing the secular rule of law to bring the cycle of bloodshed to an end.
The playwright Sophocles (496–406 BC), in Oedipus King, retold how the legendary character Oedipus received a prophecy from the oracle at Delphi, stating that since his father had been cursed, he would marry his mother and kill his father. In an attempt to outsmart the gods, he moved away from what he believed to be his parents to the city of Thebes. Along the way, he got into a fight and killed a man. Living in Thebes, he became king and married. But then, he discovered that he was adopted. His parents had abandoned him on a mountain top after hearing the same prophecy, but a shepherd had found him. It turned out that the man he had killed was his father and his Theban wife was his mother. Disturbed by this, he gouged out his own eyes and began to wander the land.
Before the discovery, Oedipus’s wife (and mother) ironically insisted that she outsmarted Apollo, a message that was amplified by the chorus:
No more shall I go in reverence to the untouchable belly-button of the world [the Oracle at Delphi]. […] The old prophecies […] are fading and now men give them no value. Nowhere is Apollo glorified with honors. Religion is no more. 
The playwright Euripides (c. 480–406 BC), a generation later, also experimented with serious atheistic ideas, although, here too, the atheists lose in the end. In The Madness of Herakles, the father of Herakles complained that Zeus had not done enough to protect his son. Based on his actions, he concluded that Zeus must be either “stupid” or “unjust.” He told Zeus, “I am only a mortal, but I outdo you in virtue.”
In his play Bellerophon, we are introduced to the Greek hero Bellerophon, who belongs to a legendary family, the house of Aeolus, who had a reputation for fighting the gods (theomakhia). Their family tree is given in a 6th century BC text called the Catalogue of Women, which is structured around a number of women impregnated by the gods. Bellerophon was one of the descendants of Aeolus. Already in Homer, we read that he was “hateful towards the gods” and “wandered […] avoiding the footsteps of humans.” In later mythology, he stole the winged horse Pegasus in an attempt to join the gods on Mount Olympus, which angered the gods. For the flight of Pegasus on the theater stage, Euripides likely used a crane, since the comic Aristophanes (c. 448–380 BC) later used the same device in a parody of the story, where a character flies to Olympus, but this time on a dung beetle (while having the actor openly call out the crane operator for moving him around too erratically). Unfortunately, only fragments of Bellerophon remain, but among them we find one of the most explicitly atheistic quotes from the ancient world:
Is there anyone who thinks there are gods above? There are not; no, there are not. Let no fool, led by the old false fable, thus deceive you. 
Fig. 313 – Bellerophon on Pegasus spears the Chimera (c. 420 BC) (Marsyas, CC BY-SA 2.5)
The atheistic tendencies of Euripides were also noted by Aristophanes, who in his play Thesmophoriazousai had a seller of religious objects complain that Euripides had put her out of business by “persuading all the men that there aren’t any gods.”
In the Sisyphus fragment, attributed in antiquity to both Euripides and Critias, the character Sisyphus, also from the House of Aeolus, explained religion as a social construct. He explains that life before religion was chaotic and violent. Then, the people set up laws “so that justice should be tyrant.” But eventually, some people figured out how to commit crimes in secret. The solution was to invent religion:
Then some shrewd man, wise in counsel, discovered for mortals fear of the gods. […] So he thereupon introduced religion, namely the idea that there is a deity […] who will hear everything said among mortals, and will be able to see everything that is done. If you plan some bad act in silence, the gods will not fail to notice. 
The comedian Aristophanes (c. 448–380 BC) also had some atheistic ideas play out on his stage. In The Knights, we read of two slaves discussing whether the gods exist:
First slave: Do you really believe in gods?
Second slave: Of course.
First slave: What’s your proof?
Second slave: The fact that I’m cursed by them. Won’t that do?
First slave: Well, it’s good enough for me.
In Clouds, Aristophanes satirized the philosopher Socrates. Unfairly, he depicted Socrates as one of the pre-Socratic philosophers, who were known for their (often baseless) speculations about the cosmos. Satirizing the pre-Socratic overemphasis on reason, he had Socrates reject traditional morality, reject Zeus on the basis of “evidence”, and accept the clouds as the only true gods:
“[The clouds] are the only true deities, all the rest are nonsense.” “What? By Earth! You don’t count Olympian Zeus as a god?” “What do you mean, Zeus? Stop babbling: Zeus doesn’t even exist!” 
The story of The Clouds tells of a deluded man who sent his gambling-addicted son to Socrates’s school hoping he would learn rhetorical tricks to talk himself out of his debts. The plan backfired when his son was taught to reject traditional values, including respect for his own father. Angry about what happened to his son, the father burned the school to the ground.
Although funny, the play didn’t help Socrates when he was taken to court. Plato, in his Apology, even blamed Aristophanes for setting the people of Athens up against him.
Various Athenian schools of philosophy from the 4th and 3rd century BC also regularly flirted with religious criticism (although this wasn’t the primary focus of these schools). The Cynics were an eccentric bunch who ignored social conventions, including conventional desires for wealth, fame, power, and possessions, which they deemed in conflict with nature and reason. In line with this, they also regularly flouted religious traditions. The most famous Cynic was Diogenes (410–323 BC), who was sometimes described as “Socrates gone mad”. He begged for a living, owned only a woolen cloak, a begging bowl, and a staff, and often slept in a large ceramic jar. He was so averse to shame that he even masturbated in public, claiming “if only it were possible to relieve hunger so easily.” When living in Athens, he criticized many of the cultural conventions of the city. He also criticized Plato’s interpretation of the teachings of Socrates and sabotaged his lectures, for instance by eating too loudly. On one occasion, Plato was applauded for his definition of a man as a “featherless biped,” after which Diogenes “plucked the feathers from a chicken, brought it to Plato’s school, and said, ‘here is Plato’s man.’”  (Aristotle would later propose the much more sensible definition of a human being as a “rational animal”). Diogenes was also critical of religious beliefs. For instance, he mocked rituals for their ineffectiveness. The Cynic poet Cercidas (3rd century BC) had similar opinions. He noted that the gods often failed to punish the wicked and reward the good, and finally concluded it “better to leave all these things to those who gaze on the heavens” and to instead “let our concern be worldly.”
The Skeptics, inspired by Socrates’s method of questioning everything, resisted dogma of any kind. They claimed there exist counterarguments to any belief and concluded from this that we cannot be certain about anything. Pyrrho (c. 360–270 BC), the founder of the Skeptics, had joined the other philosophers in the search for the secret to a happy life, but grew tired of the endeavor and finally decided to “suspend judgment” on the matter. After doing this, he was surprised to find “ataraxia,” the inner peace he had been looking for.
Pyrrho applied his Skepticism in his daily life. For instance, when at sea in a storm, he stayed unfazed, while others panicked. He also didn’t flinch during a medical procedure, wondering whether pain was truly bad. Similarly, he wondered whether washing pigs was an inferior job. In his view, external experiences were themselves neutral. It is our interpretation of them that makes them good or bad. It is therefore sometimes better to have no convictions at all.
Skepticism became popular in Plato’s Academy in the generations after Plato’s death. Carneades (c. 214–129 BC), at one point the head of the Academy, applied skepticism to the existence of the gods. The same goes for Clitomachus (c. 187–110 BC), also a head of the Academy, who wrote a now lost work called On Atheism in which he gave a history of religious skepticism up to his day.
Much later, the Skeptic Sextus Empiricus (2nd century AD) wrote a work called Against the Mathematicians, in which he critiqued various branches of philosophy and science. A part of this work is dedicated to arguments for and against the existence of gods. For instance, he reasoned that a god that is both good and omnipotent could not possibly exist in a world with so much unnecessary suffering. It would mean that he is either not capable of making the world a better place, and therefore not omnipotent, or he doesn’t want to, and therefore not good. After going through all arguments, he concluded that since “the arguments on either side are equally strong, the gods exist no more than they do not.” 
Only Aetius (2nd century AD), in a few pages of his work Placita, presented atheistic arguments without any excuse, without presenting counterarguments, and without labeling the arguments as morally objectionable (although he does not call himself an atheist).
The Epicureans, founded by Epicurus (341–270 BC), formed the most radical school of philosophy. Epicurus started a school known as The Garden (kepos) just outside the city walls of Athens. There, he sought to live an untroubled life, without stress and conflict, based on sober living and the application of reason. The Epicureans were followers of Democritus, who had claimed that everything in the world is made of atoms. This, they claimed, was even true for the soul. After death, the atoms of the soul would simply disperse, leaving no room for an afterlife. Epicurus also saw no need for a creator, but instead believed that the world was made by the chance collision of endless eternal particles. Because of these ideas, the Epicureans were often called atheists. This label was strongly resisted by Epicurus himself as he did believe in gods, although in his view they too are made of atoms and do not intervene in the lives of human beings. The Epicurean gods also had no interest in rituals, although Epicurus recommended his students to participate in the rituals of the city to not upset their fellow citizens.
Epicurus’s greatest follower was the Roman philosopher Lucretius (c. 100–55 BC). His poetic masterpiece, On the Nature of Things, introduced the Romans to the Epicurean worldview and would later be influential in secularizing the West after the Middle Ages. The text offers a strong critique of religion, calling it irrational and harmful (he referred, for instance, to Agamemnon, who sacrificed his own daughter to the goddess Artemis, in order to gain favorable winds at sea). To Lucretius, Epicurus was a great hero leading an assault on religion:
When human life lay on the ground, foully oppressed for all to see under the weight of Religion, […] then first a Grecian man [Epicurus] dared to raise his mortal eyes to meet hers—the first to dare to confront her. […] Neither the stories of the gods nor thunder nor heaven […] deterred him […] and so Religion now in turn lies beneath our feet, trampled, and his victory raises us to heaven. 
He even compared Epicurus to the giants from Greek mythology who attacked the gods in order to take over heaven (“gigantomachy”), but while the giants failed, Epicurus “brought the ramparts of the world tumbling down with reason.”
Ironically, Lucretius purposefully wrote his text in a poetic form traditionally associated with theology and even compared Epicurus himself to a god.
Much later, the Greek satirist Lucian (c. 120–180
AD), from Roman-ruled Syria, used humor to play out the atheistic
tendencies of the philosophical schools. In Zeus Refuted, Zeus is
confronted by a clever Cynic who exposes the irrational aspects of his
mythology. For instance, how can Zeus be all-powerful, while in the Iliad,
he does not have the power to save his own son Sarpedon? And if our lives are
predetermined by the gods, how can we be morally responsible? Hilariously, Zeus
has no answers to these questions and ends the conversation with a childish
insult: “You are an overbold Sophist. I’m not going to put up with this
anymore.”  In Zeus the Tragedian, we find Zeus lamenting his fate
in tragic style. Eventually, the audience figures out that the cause of his
despair is that he overheard a Stoic and an Epicurean debating whether the gods
can intervene in the world or whether they even exist. To his horror, he found
that the Epicurean was winning. Even the gods had to acknowledge the power of
his arguments but were consoled by the fact that commoners were unlikely to