G. Reale, A History of Ancient Philosophy: The Systems of the Hellenistic Age, trans. John R. Catan (Albany: State University of New York Press, 1985) 147, 157-160.
Epicurus was a philosopher who lived in the fourth-third centuries B.C.E, and he believed that people should live in pleasure. He was a critic of popular religion, for he thought that it encouraged anxiety rather than joy, plus he didn't think that the gods cared much about the affairs of humans. For Epicurus, the world came into being by chance, not through the design of a beneficent God or Demiurge. Moreover, Epicurus maintained that knowledge came primarily through the senses: what we can see, touch, hear, feel, and taste. And yet, at the same time, Epicurus believed in the existence of the gods.
Why did Epicurus believe in the gods, beings whom we cannot directly sense? One of Epicurus' reasons was that everyone in all times and places (even the uncivilized) believes in a god--or so Epicurus assumed. For Epicurus, one has to account for such a "precognition" in some way, and Epicurus concludes that "it can only come from [its] respective realities [(the gods)] even if these are outside of the range of our senses" (157). According to Cicero, Epicurus maintained that "the Gods must belong to us because nature itself has impressed the notion of them on the souls of all men" (157).
Epicurus also concluded that there were gods on the basis of certain presuppositions, which he may have reasoned out at some point. For example, he thought that, since there are many human beings and forces of destruction, then there must be an equal number of divine forces of preservation, namely, gods. Epicurus believed that the cosmos has some kind of equilibrium, which necessitates the existence of divine beings.
Epicurus also noticed that there are various degrees in the world, as some people are greater than others. Epicurus concluded from this that there must be supreme beings, who are superior to all else. These supreme beings are the gods.
Epicurus held that human beings should imitate the gods, not because the gods cared about them, but rather because they exemplified the blissful life that Epicureans valued. The gods "lived an eternal life free of worry and disturbance [and] enjoyed profound conversations in the company of friends" (159). For Epicureans, humans would be happy if they arrived at such a state.
Something I'm unclear on: to what extent did Epicureans believe that the gods were involved in human affairs? I've read in various places (including Reale) that they really did not think the gods were involved. And yet Cicero seems to say that Epicurus regarded the gods as forces of preservation. Did Epicurus think that the gods played a role in preserving the cosmos?
Also, Epicurus holds that the gods exist because virtually everyone believes in them (as far as he knows, of course). But many who believe in gods think that those gods are concerned about human beings and involved in their affairs. How would people sense the gods (as Epicurus says they do, albeit indirectly) if the gods were totally aloof from them?