So here is Democritus in the fifth century B.C. — he who coined the term “atom,” from the Greek for “indivisible,” speculating that reality consisted of nothing but fundamental particles swirling randomly around in the void — propounding an anthropological theory of the origins of religious beliefs. Talk of “the gods,” he argued, comes naturally to primitive people who, unable yet to grasp the laws of nature, resort to fantastical storytelling. The exact titles of his works remain in doubt, but his naturalist explanation of the origins of conventional religion might have made use of Daniel C. Dennett’s title “Breaking the Spell: Religion as a Natural Phenomenon.”

Or take the inflammatory title of Christopher Hitchens’s book, “God Is Not Great: How Religion Poisons Everything.” Lucretius, who lived in the first century B.C., chose a more neutral title for his magnificent poem, “De Rerum Natura,” or “On the Nature of Things,” but he concurred with the sentiment expressed in Hitchens’s subtitle. He focused not just on the groundlessness of beliefs proffered in ignorance of the natural causes of physical phenomena, but also on their behavioral consequences. In the grip of religious conviction, a person will commit acts too horrific to otherwise contemplate. So Agamemnon, advised by a priest, made a human sacrifice of his daughter to appease the goddess Artemis, who had been offended over the killing of a deer. “Tantum religio potuit suadere malorum,” Lucretius wrote: “Such is the terrible evil that religion was able to induce.” Though the religion may have changed, the point remained sufficiently pertinent for Voltaire to quote the line to Frederick II of Prussia in urging the case for secularism.

But whereas Lucretius focused on the immorality of men when under their religious delusions, other ancients stressed the immorality of the gods themselves, who either passively permit or actively participate in human tragedies. The gods are not great. Euripides, toward the end of his life, composed “The Madness of Heracles,” which has one character dressing down Zeus: “You are a stupid kind of god, or by nature you are unjust.” Mortals morally overtake immortals, the gods being oblivious to what the virtuous know: the value of human life, the outrage of its guiltless suffering.

And then there are those pre-­Socratics, like Xenophanes and Anaxagoras, who, in my mind, foreshadow what would be Spinoza’s special brand of atheism, identifying God with nature — or, more specifically, the intelligible structure of nature expressed in unchangeable laws. “Xenophanes, then, was not an atheist in any straightforward sense,” Whitmarsh writes. “He was not denying the existence of deity but radically redefining it.” The author goes on to ask whether anything would be lost “in Xenophanes’ account of the world if we substituted ‘nature’ for ‘the one god.’ ” Such a redefinition reappears not only in Spinoza’s magnum opus, the posthumously published Ethics, but in those who studied Spinoza, including Einstein. When asked whether he believed in God, Einstein responded, “I believe in Spinoza’s God,” which amounted to an affirmation of the guiding principle of science, namely nature’s beautiful intelligibility.