Epicurus was the greatest philosopher of all time.

He single-handedly foreshadowed virtually all of modern science. He described the basics of Darwinian natural selection over 2,000 years before Darwin. He stated the law of inertia 2,000 years before Galileo. He understood that the Universe has no absolute center and believed in the existence of exoplanets over 2,000 years before modern cosmology. He thought the stars were just physical systems that come and go, instead of divine and eternal beings, like almost all people believed in the ancient world. He revolutionized and reconceptualized the atomic theory inherited from Democritus and Leucippus and gave us the ultimate cohesive version of it that would profoundly impact scientists like Robert Boyle and Isaac Newton, accelerating the Scientific Revolution as a result. Newton in particular was deeply influenced by Epicurus. Newton's theory of light was just a spruced up version of Epicurean ideas (through which he explained phenomena like reflection and refraction), part of a broader wave of Christian atomism that spread like wildfire in the 17th century, spearheaded by Gassendi, Charleton, and Boyle. Thomas Jefferson considered himself an Epicurean, and Karl Marx wrote his doctoral dissertation on Epicurean materialism, serving as a jumping point for the broader historical materialism he would come to embrace and articulate. Epicurean influence on humanity is difficult to overstate.

Epicureanism was fundamentally an ethical philosophy aimed towards achieving ataraxia, or the absence of fear and anxiety in the mind, thereby leading to mental tranquility in the face of the world's chaos (Buddhists will rightly flay me for this comparison, but you can think of ataraxia as a more down-to-earth version of nirvana). For Epicurus, knowing about how the natural world actually worked was extremely important for getting to ataraxia and ethical integrity, chiefly because he ascribed much of the world's ills to the fear of death, which convinced people to do and believe stupid things, according to him. By learning about nature, we could overcome our fear of death and live a life free of anxiety.

Epicurus was a philosophical giant who received religious levels of reverence in the five centuries after his death, actually exceeding Plato, Aristotle, and the Stoics in popularity for long periods of time. In a biographical anthology of famous Greek philosophers from the 3rd century (the only surviving source from ancient times containing critical biographical details about these figures, like where they were born and who their parents were), the writer Diogenes Laertius gives Aristotle and his philosophy roughly 15 measly pages tucked away in a book with a list of other philosophers. There are only two thinkers that receive their own books in the anthology: Plato and Epicurus. And this isn't an isolated phenomenon because Diogenes may have been pro-Epicurean. Even Sextus Empiricus, in the Outlines of Pyrrhonism, goes on to mention Epicurus more often than Aristotle. The same is true for many other ancient philosophical texts from the 1st and 2nd centuries. Epicurus was famous or infamous enough (depending on your point of view) that the word "apikoros" (almost certainly derived from his name) became the standard term for "heretic" in the Jewish tradition. The events of The Garden, the school founded by Epicurus, even carried political implications. In 121 CE, Trajan's widow Plotina (Trajan had died four years earlier) wrote to Hadrian and requested that he approve new rules for the succession of the scholarch at The Garden, to which Hadrian agreed. Millions of common people were Epicureans, but so were many powerful people, including senators and aristocrats that lavishly patronized numerous Epicurean thinkers and writers (including, of course, the likes of Lucretius and Philodemus, the latter being patronized by Piso, Caesar's father-in-law). 

Lucretius intimated that Epicurus was a god at one point in The Nature of Things. Epicurus had a huge following in Antiquity that bordered on religious fervor, which is funny because he was a materialist. Epicureans would gather on the Eikas ("Twentieth") of each month and celebrate Epicurus and his philosophy through feasts, games, and readings. Since Epicurus was held to have been born on February 20 (in 341 BCE), February 20 of every year became something like the Epicurean Christmas, a big annual bash where Epicureans, normally known for championing simplicity in life, would go all out in their celebrations. Epicureans even carried a modified version of the Tetrapharmakos (the fourt-part cure, or four short doctrinal sayings that summarized the ethical dimensions of Epicurean philosophy) to their grave. Before the cross, legions of Roman tombstones in Antiquity, for slaves and gladiators alike, were inscribed with NFFNSNC, short for "Non fui, fui, non sumo, non curo" ("I was not, I was, I am not, I don't care"). The full Tetrapharmakos read:

1) Don't fear the gods (because, according to Epicurus, they're natural systems composed of atoms and have no interest in human affairs)

2) Don't worry about death (because you won't feel or experience anything while you're dead)

3) What's good is easy to get (friendship, basic necessities, etc)

4) What is terrible is easy to endure (very painful things go away quickly and pain that endures can be managed)

We know Epicureanism was a powerful philosophical school even into the 3rd century CE, given multiple lines of evidence, including the insufferable amount of ink that Plotinus spills in The Enneads for attacking atomism and materialism. The school's influence almost certainly waned by the 4th century CE, not just because of the rise of Christianity, but also because of broader economic and political changes happening in the Roman Empire in Late Antiquity. By the Middle Ages, it was Plato and Aristotle who came to dominate the Western philosophical canon. That Plato and Aristotle became so popular in medieval times has less to do with their Christian compatibility, as is commonly supposed, and more to do with the fact they were philosophers of empire, so their works had already circulated in the Eastern Mediterranean long before anyone knew who Jesus was. Alexander the Great heavily patronized Aristotle and gave him the equivalent of millions of dollars to expand his library in Athens; Alexander also authorized the transmission of many Aristotelian works in various parts of his new Hellenistic empire, which immediately collapsed after his death as rival generals descended into a series of interminable wars. It was because of that political association that Aristotle fled Athens shortly after news of Alexander's death arrived, because most Athenians saw him as an imperial stooge and he likely faced execution if he had chosen to stay behind. And so this was the main reason why so much of Aristotle's written corpus survived in the Middle East and got picked up by Islamic scholars, who preserved it long enough that it could eventually make its way back to the Western world in the Middle Ages. 

Epicurus wasn't as lucky. The vast majority of his works, and he wrote something like 300 books, have not survived in full. We only have three complete letters from him and a whole bunch of fragments from various sources, including an Epicurean library preserved at Herculaneum after the eruption of Vesuvius in 79 CE. In the 15th century, the sensational rediscovery of The Nature of Things (my personal Bible, a lengthy didactic poem by Lucretius meant to summarize On Nature, the central masterpiece from Epicurus) instantly revived intellectual interest in Epicurus, and the rest, as they say, is history.