Epicurus was not an "atheist"

That he believed in Gods

Epicurus was born in Samos, Greece in 341 BC and he believed in the Gods. In modern times he is described as an atheist but this is simply not true and there is plenty of textual evidence to back this up such as

"First, believe that god is a blissful, immortal being, as is commonly held. Do not ascribe to god anything that is inconsistent with immortality and blissfulness; instead, believe about god everything that can support immortality and blissfulness. For gods there are: our knowledge of them is clear. Yet they are not such as most people believe; indeed most people are not even consistent in what they believe. It is not impious to deny the gods that most people believe in, but to ascribe to the gods what most people believe. The things that most people say about the gods are based on false assumptions, not a firm grasp of the facts [note], because they say that the greatest goods and the greatest harms come from the gods. For since they are at home with what is best about themselves, they accept that which is similar and consider alien that which is different."

No he was not an atheist but his views of the gods were radically different than his countrymen and indeed even modern theists.

The gods are not supernatural

For Epicurus the gods were not outside of the laws of the universe, they were made up of the same things which humans and everything else was, atoms and void. 

Why is this important?

Even though he was not an atheist, he was a naturalist meaning that only natural laws or forces operate the universe, rather than supernatural or spiritual laws or forces. 

Denying supernatural or spiritual forces or laws and accepting natural laws or forces means that if there is a God or gods, they must be able to be explained within the realm of what is natural within the universe. They do not rely on "belief" or "faith" but rather evidence.

He also thought that the gods didn't effect our lives in the slightest, they blessed beings who live in state of complete pleasure.