Like most peoples of the ancient world, the Greeks were polytheists, worshipping a variety of gods and goddesses who were immortal but otherwise acted just like people. As elsewhere, Greek religion was primarily a matter of ritual, with rituals designed to appease the divinities believed to control the forces of the natural world. Processions, festivals, and sacrifices offered to the gods were frequently occasions for people to meet together socially. Migration, invasion, and colonization brought the Greeks into contact with other peoples and caused their religious beliefs to evolve.

By the classical era, the primary gods were understood to live metaphorically on Mount Olympus, the highest mountain in Greece. Besides these Olympian gods, each polis had its own minor deities, each with his or her own local group of worshippers. The polis administered the cults and religious festivals, and everyone was expected to participate in these civic rituals. In contrast to Mesopotamia, Egypt, and Vedic India, priests held little power in Greece; their purpose was to care for temples and sacred property and to conduct the proper rituals, but not to make religious or political rules or doctrines. Much religion was local and domestic, and individual families honored various deities privately in their homes. Many people also believed that magic rituals and spells were effective and sought the assistance of individuals reputed to have special knowledge or powers.

Along with public and family forms of honoring the gods, some Greeks also participated in what later historians have termed mystery religions, in which participants underwent an initiation ritual and gained secret knowledge that they were forbidden to reveal to the uninitiated. Many of these religions promised rebirth or an afterlife to adherents.

The Greeks also shared some Pan-Hellenic festivals, the chief of which were held at Olympia to honor the god Zeus and at Delphi to honor the god Apollo. The festivities at Olympia included athletic contests that inspired the modern Olympic games. Held every four years after they started in 776 B.C.E., the contests attracted visitors from all over the Greek world. The Pythian games at Delphi were also held every four years, and these contests included musical and literary competitions.