Although its exact origins are lost in time, Greek religion is thought to date from about the period of the Aryan invasions of the 2d millennium B.C. Those invaders encountered two other peoples who had existed in the region of Greece from Neolithic times: the Aegeans (Pelasgians) and the Minoans of Crete. The Aryans fused with the Aegean and Minoan cultures to create what is now considered Greek culture. The result, known as the Minoan-Mycenean civilization, flourished in the period from 1600 B.C. to 1400 B.C.
Previous to the invasions, the Helladic communities had been widely separated geographically, but the attacking foreigners swept everything along in their path, including various beliefs that were prevalent in the outlying districts. At first the result was a confused conglomeration, but gradually a certain systematization of the gods began to take place. The marriage of Zeus, a sky god of the conquerors, and Hera, a fertility goddess of the conquered, symbolized the attempt at fusion, although the constant conflict between the divine pair, as seen in the Iliad, indicates the tensions of the match. The classical Greek pantheon was peopled with gods from all the cultures involved: Zeus the sky father, Demeter the earth mother, and Hestia, the virgin goddess of the hearth, were borrowed from the Indo-European invaders; Rhea was an indigenous Minoan goddess; Athena was Mycenean; Hera and Hermes were Aegean; Apollo was Ionian; Aphrodite came from Cyprus and Dionysus and Ares from Thrace.
Just before the violent Doric invasions, the Achaeans fought the Trojans of Asia Minor. The chronicle of that war, the Iliad, furnishes the first clear picture of the early Greek religion as it evolved from a blending of Achaean, Dorian, Minoan, Egyptian, and Asian elements. This phase of Greek religion is called Homeric, after the author of the Iliad, or Olympian, after Mount Olympus, the Thessalian mountain where the gods dwelled. The early Egyptian influences represented by half-human, half-animal deities vanished, and the Olympians were purely anthropomorphic figures. Zeus was the supreme lord of the skies, retaining his original Aryan importance; he shared his dominion with his two chthonic and pre-Aryan brothers, Hades, lord of the underworld, and Poseidon, lord of the waters.
Through a vast set of myths and legends (the clearest illustration is Hesiod's Theogony) the other gods and goddesses were carefully related to one another until a divine family was established with Zeus as its titular head. The Homeric pantheon was a tightly knit family group in charge of natural forces but not equal to the natural forces themselves. The gods had supernatural powers (particularly over human life), but their power was severely limited by a concept of fate (Moira) as the relentless force of destiny. The gods were not thought to be omnipresent, omniscient, or omnipotent. Shorn of the usual godly attributes, the Olympians often took on the property of being simply bigger than humans, but not different or alien. The Olympians fought one another and often meddled in human affairs (this intervention was called the deus ex machina, or divine intervention).
The superhuman features of the Olympians were their immortality and their ability to reveal the future to humanity. The Greeks did not consider immortality a particularly enviable property. Action was crucial and exciting by the very fact of life's brevity, and people were expected to perform by their own particular heroic arete, or virtue. Death was a necessary evil; the dead were impotent shades without consciousness, and there are only vague images of the Isles of the Blest in an Olympian world. The Greeks, however, did expect information about their future life on earth from the gods. Thus divination was a central aspect of religious life (see oracle).
The Olympians were, perhaps, most important in their role as civic deities, and each of the Greek city-states came to consider one or more of the gods as its particular guardian. There were public cults that were devoted to insuring the city against plague, conquest, or want. The religious festival became the occasion for a great assembly of citizens and foreigners.
The civil strife that followed the classical period (from c.500 B.C.) placed the old gods on trial. Often the gods did not answer with the visible and immediate rewards that were expected. Although the Homeric gods had distinctive personalities, their reality still had to be accepted intellectually. This form of religion suited the sophisticated city dwellers, among whom there was even a strong monotheistic tendency; however, it did not meet the needs of the people of the provinces, the farmers and shepherds, who retained primitive notions steeped in superstition (see animism).
Once the gods were placed on trial, the door was open for the popular religion of the Greek countryside. Since the gods could no longer be trusted to make life agreeable, an emphasis was placed on regeneration and on the afterlife. The mysteries gained importance after Homeric religion was established, but the origins in the seasonal festivals that underlie many of them go back as far as 1400 B.C. The Eleusinian Mysteries were perhaps the most widely practiced of the mysteries. Other popular rites were the mysteries of Dionysus and the Orphic Mysteries.
In reaction to Dionysian excesses, Apollo eventually appropriated many of the virtues of the older gods, such as justice, harmony, legalism, and moderation. The tension between the Apollonian and Dionysian strains was particularly illustrated in the work of the tragic poets of Greece, the dramatists such as Aeschylus, Sophocles, and Euripides, who had begun to question the justice and integrity of the gods.
It was in the area of philosophical thought that a clear-cut monism developed to augment and also shift Greek religious thought to a new type of speculation. The Greek philosophers sought a more rational and scientific approach in humanity's relation to nature, espousing a logical and important connection between humanity and nature, not a mysterious and secret one between humans and god. It was Plato who made an absolute abstraction of the highest virtue, giving to that abstraction the quality of Absolute Good to which even the gods must be true. Philosophical inquiry led to the rationalization of myths and completed the destruction of the Homeric pantheon. The vacuum was eventually filled by Christianity.
See M. P. Nilsson, A History of Greek Religion (tr. 1925, 2d ed. 1964); W. W. Jaeger, Paideia (4 books in 3 vol., 1939-45, repr. 1960-62); W. K. C. Guthrie, The Greeks and Their Gods (1950, repr. 1956); R. Graves, The Greek Myths (2 vol., 1955, repr. 1959); B. C. Dietrich, The Origins of Greek Religion (1974); W. Burhert, Greek Religion (1985); A. H. Armstrong, ed., Classical Mediterranean Spirituality (1986).