Were the ancient Greeks unbelievers and secularists?
I recommend the book heartily. Still it offers no direct answer to the provocative question it poses. Before approaching the matter more closely, we must first pose two additional questions: how much? and when? That is, in Homeric times and several centuries thereafter, the Greeks generally accepted the reality of the gods--that they lived eternal happy lives (sometimes scandalous ones), and did not scruple to interfere in human affairs. Beginning in the sixth century BCE, however, a number of intellectuals began to question the conventional views, sometimes in ways that were dangerous to their health. As a rule, such questioning must be distinguished from today's "strong" atheism, which simply denies the existence of the gods altogether.
In many instances the Greeks used the adjective atheos (ἄθεος), ungodly, as a synonym for asebēs (ἀσεβής), impious. As an abstract noun, there was also atheotēs (ἀθεότης). (The form "atheism" is modern.)
In the popular view one should not disrespect the gods; that could be dangerous to the community when the disrespectees took offense. There are of course various types of disrespect, including questioning, mockery, limitation of the deities' powers, and of course outright, categorical denial--atheism in the modern sense.
Many scholars regard the Pre-Socratic thinkers, beginning with Thales of Miletus (mid-620s-mid-540s BCE), as having taken the first steps in dismantling Greek polytheism by shifting from the idea that the gods interfere freely in human affairs (as in Homer) to explanations of change through natural causes. Insofar as we can reconstruct Thales' ideas from the few extant fragmenrts, it is true that he does tend to look to natural causes. He held that the fundamental element was water.
However, Aristotle reports that Thales affirmed that “All things are full of gods.” There are two standard explanations for this dictum (explanations which may of course be combined). 1) The gods Thales mentions are not the highly anthropomorphic figures that appear in Homer and Hesiod, but rather more like powers or forces, stripped (to some degree, at least) of personalization. 2) The gods are closely connected to, if not identified with, the "soul" that Thales attributes to the magnet. On this basis some scholars interpret Thales’ claim as a manifestation or development of the animistic tendencies present in many human societies. It is a common primitive tendency to regard rivers, trees, and so on as somehow animated or inhabited by spirits. This point suggests that Thales' outlook was not as “modern” as some of his latter-day admirers like to think.
The contrast between the natural-causes tendency and the idea that world is full of gods may simply reflect the fact that ancient Greek thinkers, like many people today, were capable of entertaining two contradictory ideas at once--in other words, what is termed cognitive dissonance. This conflict has been detected in the poet Hesiod (eighth century?), who points to natural occurrences as signaling the times when one should plant and harvest crops. "When the Pleiades, daughters of Atlas, are rising, begin your harvest, and your ploughing when they are going to set. (Hesiod, Works and Days 385)" In his Theogony, however, Hesiod provides elaborate accounts of the gods.
At all events, Thales and Hesiod and their contemporaries show evidence of weighing different sorts of explanations, and this weighing takes us some distance along the road to skepticism. Still it should be emphasized that even a whole-hearted extraction of the gods from our world does not imply that they do not exist. They may in fact be thriving in their own sphere, but indifferent to human concerns and fates.
Another advance is attributed to Xenophanes of Colophon (ca. 570 – 480 BCE), a Greek philosopher, poet, and social and religious critic. To judge from the surviving fragments of his poetry, hie criticized and satirized a wide range of ideas, including Homer and Hesiod, the belief in anthropomorphic gods, and the Greeks' veneration of athleticism. Xenophanes rejected the idea that the gods resembled humans in form, anticipating, in the view of some scholars, the skepticism that became current during the fourth century. A famous passage ridiculed the idea by claiming that, if oxen were able to imagine gods, then those gods would be in the image of oxen: “The Ethiopians say that their gods are flat-nosed and black, while the Thracians say that theirs have blue eyes and red hair. Yet if cattle or horses or lions had hands and could draw, and could sculpt like men, then the horses would draw their gods like horses, and cattle like cattle; and each they would shape bodies of gods in the likeness, each kind, of their own.”
Notice that Xenophanes assigns an important role to painting and sculpture in reinforcing the anthopomorphic concept of the gods. Eventually, this critique was to lead to a tendency to avoid religious images altogether. At all events his relativism does not amount to an outright denial of the existence of the gods.
Because of his development of the concept of a "one god greatest among gods and men" who is abstract, universal, unchanging, immobile and always present, Xenophanes is often seen as one of the first monotheists. This view is unjustified, however, because he is talking about one supreme god among other deities, whose collective existence he does not deny.
Others had become uneasy about the all-too-human escapades that mythology attributes to the gods, including their amours. This uneasiness undoubtedly helped promote a more abstract and generalized notion of the godhead (ho theos), as found in Plato and Aristotle.
The fifth-century BCE Greek thinker Diagoras has been hailed as the "first atheist.” To judge from the very fragmentary evidence, he strongly criticized existing practices in religion and mysticism. Yet it is not clear that he denied outright the existence of the gods. Critias advanced a very influential view, in that he regarded religion as an invention used to frighten people into following moral order. Again, this critique is aimed at the abuses of traditional religion, without necessarily implying that there could be no divine principle at all.
Atomists ,such as Democritus attempted to explain the world in a purely materialistic way, without reference to the spiritual or mystical. But once more, narrowing the sphere of the gods does not extinguish them.
Socrates (c. 471–399 BCE), was accused of impiety (asebeia) because he was supposed to have enouraged questioning of the state gods. Although he disputed the accusation that he was a "complete atheist,” saying that he could not be an atheist as he believed in spirits, he was ultimately sentenced to death. Socrates prays to various gods in Plato's dialogue Phaedrus and swears "By Zeus" in The Republic.
Euhemerus (c. 330–260 BCE) advanced another influential view. He held that the traditional gods were only the deified rulers, conquerors and founders of the past, and that their cults were in essence the continuation of the state propaganda of vanished kingdoms.
The atomic materialist Epicurus (c. 341–270 BCE) disputed many religious doctrines, including the existence of an afterlife or a personal deity; he considered the soul purely material and mortal. He and his school, the Epicureans, did not rule out the existence of gods. Yet they believed that if they did exist, they were unconcerned with humanity.
The Roman poet Lucretius (c. 99–55 BCE), an Epicurean, agreed that, if there were gods, they were indifferent to humanity and unable to affect the natural world. For this reason, he counseled that humanity should have no fear of the supernatural.
The Roman philosopher Sextus Empiricus held that one should suspend judgment about virtually all beliefs—a form of skepticism known as Pyrrhonism—that nothing was inherently evil, and that ataraxia ("peace of mind") is attainable by withholding one's judgment.
Later, pagan intellectuals labeled the early Christians atheists because of their disbelief in pagan gods.
This last instance reinforces the point that one should be cautious about assigning modern meanings to the Greek (and Roman) terms translated as "atheist.” Often this concept was simply equivalent to “asebeia,” impiety or mockery of the traditional gods.
Walter Burkert, the greatest living authority on Greek religion, has stated that the beliefs of the ancient Greeks were very strange, and are likely to remain so. For this reason one should be careful about retrojecting our own ideas about Enlightenment, skepticism, secularism, and religious unbelief to this remote, but influential era of human culture.