The origin of Greek mythology and beliefs about the gods must go way back in time but, from the point of view of a fifth-century Greek like Herodotus, Homer and Hesiod were effectively the founding fathers of the literary record. As Herodotus puts it in his Histories:

The polytheism apparent in the Homeric poems lasted throughout Greece essentially until well into the Christian era. Despite any tendency on the part of philosophers and thinkers like Plato and Aristotle to move to monotheism, the old gods continued to rule.