Greek religion is older than Greece's alphabet. The Linear B tablets of Mycenaean palaces, from the late Bronze Age, already name Zeus, Hera, Poseidon and Dionysus among recipients of offerings; after the palaces fell, the traditions reformed around the rising city-states, and the eighth century brought the panhellenic institutions that made the Greeks one people in religion while remaining rivals in politics: the oracle at Delphi consulted before colonies sailed, the games at Olympia dated by their four-year cycle, and the epics of Homer recited everywhere Greek was spoken.
Philosophy grew inside this world, not against it. Xenophanes mocked gods in human form, Plato rewrote theology as the pursuit of the good, and the Stoics allegorized Zeus into cosmic reason, yet the sacrifices, festivals and oracles continued unbroken; Socrates was executed for impiety by a city that took its gods seriously. Alexander's conquests carried Greek cults across the Near East, where they mingled with Egyptian and Anatolian worship into new forms, Serapis in Alexandria, ruler cult everywhere, and the mysteries multiplied.
Under Rome the old sanctuaries were honored, restored and finally closed: Theodosius ended the Olympic games and the Eleusinian mysteries at the end of the fourth century CE, and the oracles fell silent. What survived was the grammar: Greek terms, mystery, ecstasy, theology, liturgy itself, through which later religions, Christianity first among them, learned to describe the sacred.