Greek religion was something done rather than believed. Its central act, as Walter Burkert's standard study shows, was animal sacrifice: the procession, the sprinkled barley, the victim's consecrated death, the gods' portion burning on the altar while the community feasted on the rest. Around that act turned the festival calendars of every city, each polis keeping its own cycle for its own gods, Athena's Panathenaia at Athens, Hera's festivals at Argos, Dionysus' dramatic festivals from which tragedy and comedy grew.
The Olympian family gave the practice its faces: Zeus of the sky and oaths, Hera of marriage, Poseidon of the sea and earthquake, Athena of craft and counsel, Apollo of prophecy, music and purification, Dionysus of wine and ecstatic release, Demeter of grain and her daughter Persephone, whose abduction and return figured the seasons themselves. Poets, not priests, organized this theology; Homer and Hesiod, as the Greeks themselves said, gave the gods their names, honors and forms.
Between gods and mortals ran a dense traffic of communication: oracles above all Delphi, where the Pythia answered for Apollo; seers reading entrails and birds on every battlefield; and purification from pollution, miasma, whose logic Robert Parker has mapped. For deeper hopes there were the mysteries. At Eleusis, initiates of Demeter and Persephone underwent secret rites that promised a better portion after death, a secret kept so well for a thousand years that it remains unknown.