The story goes that the renowned physicist Richard Feynman was once asked to summarize the most important finding of modern science in a single sentence. Feynman replied, "The universe is made of atoms."
Although there are many other scientific discoveries that are arguably of equal importance, Feynman's choice makes a lot of sense. The discovery of atoms is so familiar to us that it's easy to overlook its breathtaking significance. We know, at the smallest scale where it still makes sense to talk about distinct objects, what are the fundamental building blocks that matter is made of, and we have described their interactions with astounding precision. Our understanding of everything from why the stars shine, to how DNA replicates, to why a table is solid, relies on our knowledge of the way atoms behave.
Atomic theory is now so well-established, and so widely accepted, that it's easy to forget how controversial a notion it originally was. In fact, atomism was once synonymous with atheism, and it was the bête noire of Western religion not just for centuries, but for millennia.
It was in the fifth century BCE that the Greek philosophers Leucippus and Democritus first proposed the idea that matter was composed of indivisible particles called atoms. But these ideas came to their fullest flowering in the mind of their successor, Epicurus, who lived around 300 BCE. In Epicurean philosophy, the world was ultimately comprised of atoms and the void. All that exists and all that occurs - from flowing water to burning fire to human thought - is due to the movement and collision of atoms and the endless, ever-changing array of patterns they arrange themselves in. The ruling principles of the Epicurean cosmos are natural law and random chance, not purpose or plan, and we who live in it and are part of it can find happiness by learning to accept whatever happens with virtue and tranquility. Epicurus did believe that the gods existed - he saw this as the only way to explain the widespread dreams and visions of them - but in his philosophy, they were not supernatural spirits but material beings composed of atoms, just like humans. More importantly, they did not take any interest in human affairs; they were more like images than actual persons.
In scientific terms, it's impressive how much Democritus and Epicurus got right. They correctly anticipated the very discovery that Richard Feynman called the most important element of modern science. Epicurus even believed that atoms sometimes exhibited "random swerves", a startling point of agreement with modern quantum mechanics. If he had claimed that a god told him all this, it would have been by far the most impressive example of theism anticipating later scientific discovery, and genuinely difficult for an atheist to explain.
Yet to the theologians and churchmen who came after him, Epicurus' ideas were the depths of heresy. His materialist notion of the cosmos - no creator deity, no life after death, everything that exists made of patterns of atoms - was anathema to the monotheist conception of an orderly cosmos arranged and guided by God. For centuries, being accused of "Epicureanism" was a very serious charge indeed. For example, the Jewish writings known as the Mishnah, in 200 CE, had this to say:
And these are the people who do not merit the world to come: The ones who say that there is no resurrection from the dead, and those who deny the Torah is from the heavens, and Epicureans.
Indeed, the Jewish word for "heretic" - apikoros - appears to be a Hebrew transliteration of "Epicurean". The Hebrew benediction known as the Amidah, which is recited three times daily by observant Jews, contains a prayer which asks that "may all the apikorsim be destroyed in an instant" (source).
As Christianity became ascendant, it treated Epicureans no less kindly. Acts 17:16-18 records how the first Christians viewed them:
"Now while Paul waited for them at Athens, his spirit was stirred in him, when he saw the city wholly given to idolatry. Therefore disputed he in the synagogue with the Jews, and with the devout persons, and in the market daily with them that met with him. Then certain philosophers of the Epicureans, and of the Stoicks, encountered him. And some said, What will this babbler say? other some, He seemeth to be a setter forth of strange gods."
Early Christian apologists such as Tertullian, Jerome and Augustine reviled Epicurus, calling him a "pig" and an advocate of "depravity and gluttony", and his philosophy a "frigid conceit" (source; see also).
Throughout the Middle Ages, as Christianity gained secular power, the ridicule and persecution grew worse. The Byzantine emperor Justinian I, who actively suppressed non-Christian faiths, closed down the philosophy schools of Athens, including the Epicurean Garden, which had survived for eight hundred years. The twelfth-century philosopher Nicholas of Autrecourt, who taught an atomist doctrine similar to Epicurus', was condemned and forced to recant and burn his writings. In the Divine Comedy, Dante depicts Epicurus and all his followers "who with the body make the spirit die" as imprisoned in flaming tombs for all eternity. As late as the 1600s, Epicurean theories were reviled, as one pamphleteer wrote: "Let that beastly Epicure's mouth be now sealed up in dumb silence."
Yet Epicurus, that sly old Greek, had the last laugh. The church persecuted his followers and sought to stamp out his teachings, but not only did Epicureanism survive, it was vindicated. The universe is made of atoms after all. Natural phenomena like weather, the growth of crystals, even the currents of human motion and thought can be traced back to patterns of atoms and their ceaseless ebb and flow. As in many other areas, this is one where religion arrogantly thought to wade in before science had had its say, and was forced to retreat. We do not live in the medieval church's world, where our bodies are just so much fleshly dust powered by immaterial currents of spirit, and the heavenly bodies move in spheres of celestial ether. We live in a grand cosmic clockwork of atoms and molecules, a vast mesh whose unfolding is determined by random chance and the immutable laws of cause and effect. We live in Epicurus' world.