History.

Greek mythology is the body of myths that were told by the ancient Greeks, people started believing in it from about 3000 to 1100 BCE. Chaos was the first Greek god that came into existence. In the past Greek myths were transmitted orally, but as years went by Hesiod and Homer, Greek poets, wrote about it.

The poet Hesiod wrote a Theogony that offered the origin story of Greek mythology. In the Theogony its written the story of the universe journey from nothingness (a void) to being, it also talks about a family tree of elements that descended from Gaia (Earth), Ouranos (Sky), Pontos (Sea) and Tartaros (the underworld).

There exists twelve main Olympians which are Zeus, Hera, Aphrodite, Apollo, Ares, Artemis, Athena, Demeter, Dionysos, Hephaistos, Hermes and Poseidon. 

"The characters, stories, themes and lessons of Greek mythology have shapped art and literature for thousands of years they appear in the Renaissance paintings such as Botticelli´s Birth of Venus and Raphael´s Triumph of Galatea and writings like Dante´s Inferno; Romantic poetry and libretti; and scores of more recent novels, plays and films."

The Greek Gods.

References: History.com Editors. "Greek mythology", History. https://www.history.com/topics/ancient-history/greek-mythology (consultation date: 09/04/19).