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Classical antiquity

Western atheism has its roots in pre-Socratic Greek philosophy, but atheism in the modern sense was extremely rare in ancient Greece. Pre-Socratic Atomists such as Democritus attempted to explain the world in a purely materialistic way and interpreted religion as a human reaction to natural phenomena, but did not explicitly deny the gods' existence.

Anaxagoras, whom Irenaeus calls "the atheist", was accused of impiety and condemned for stating that "the sun is a type of incandescent stone", an affirmation with which he tried to deny the divinity of the celestial bodies. In the late fifth century BCE, the Greek lyric poet Diagoras of Melos was sentenced to death in Athens under the charge of being a "godless person" (ἄθεος) after he made fun of the Eleusinian Mysteries, but he fled the city to escape punishment. In post-classical antiquity, philosophers such as Cicero and Sextus Empiricus described Diagoras as an "atheist" who categorically denied the existence of the gods, but in modern scholarship Marek Winiarczyk has defended the view that Diagoras was not an atheist in the modern sense, in a view that has proved influential. On the other hand, the verdict has been challenged by Tim Whitmarsh, who argues that Diagoras rejected the gods on the basis of the problem of evil, and this argument was in turn alluded to in Euripides' fragmentary play Bellerophon. A fragment from a lost Attic drama that featured Sisyphus, which has been attributed to both Critias and Euripides, claims that a clever man invented "the fear of the gods" in order to frighten people into behaving morally.

Does then anyone say there are gods in heaven? There are not, there are not, if a man is willing not to give foolish credence to the ancient story. Consider for yourselves, don't form an opinion on the basis of my words!

Protagoras has sometimes been taken to be an atheist, but rather espoused agnostic views, commenting that "Concerning the gods I am unable to discover whether they exist or not, or what they are like in form; for there are many hindrances to knowledge, the obscurity of the subject and the brevity of human life." The Athenian public associated Socrates (c. 470–399 BCE) with the trends in pre-Socratic philosophy towards naturalistic inquiry and the rejection of divine explanations for phenomena. Aristophanes ' comic play The Clouds (performed 423 BCE) portrays Socrates as teaching his students that the traditional Greek deities do not exist. Socrates was later tried and executed under the charge of not believing in the gods of the state and instead worshipping foreign gods. Socrates himself vehemently denied the charges of atheism at his trial. From a survey of these 5th-century BCE philosophers, David Sedley has concluded that none of them openly defended radical atheism, but since Classical sources clearly attest to radical atheist ideas Athens probably had an "atheist underground".

Religious skepticism continued into the Hellenistic period, and from this period the most important Greek thinker in the development of atheism was the philosopher Epicurus (c. 300 BCE). Drawing on the ideas of Democritus and the Atomists, he espoused a materialistic philosophy according to which the universe was governed by the laws of chance without the need for divine intervention (see scientific determinism). Although Epicurus still maintained that the gods existed, he believed that they were uninterested in human affairs. The aim of the Epicureans was to attain ataraxia ("peace of mind") and one important way of doing this was by exposing fear of divine wrath as irrational. The Epicureans also denied the existence of an afterlife and the need to fear divine punishment after death.

Euhemerus (c. 300 BCE) published his view that the gods were only the deified rulers and founders of the past. Although not strictly an atheist, Euhemerus was later criticized by Plutarch for having "spread atheism over the whole inhabited earth by obliterating the gods". In the 3rd century BCE, the Hellenistic philosophers Theodorus Cyrenaicus and Strato of Lampsacus were also reputed to deny the existence of the gods. The Pyrrhonist philosopher Sextus Empiricus (c. 200 CE) compiled a large number of ancient arguments against the existence of gods, recommending that one should suspend judgment regarding the matter. His relatively large volume of surviving works had a lasting influence on later philosophers.

The meaning of "atheist" changed over the course of classical antiquity. Early Christians were widely reviled as "atheists" because they did not believe in the existence of the Graeco-Roman deities. During the Roman Empire, Christians were executed for their rejection of the Roman gods in general and the Imperial cult of ancient Rome in particular. There was, however, a heavy struggle between Christians and pagans, in which each group accused the other of atheism, for not practicing the religion which they considered correct. When Christianity became the state religion of Rome under Theodosius I in 381, heresy became a punishable offense.