The study of the cults of Zeus is perhaps the most interesting chapter of the history of Greek religion, for it includes the two extremes of religious thought, the most primitive ideas side by side with the most advanced; and nearly all the departments of nature and human life were penetrated with this worship. Although the figures of Apollo, Athene, Dionysos, and Prometheus are of more importance in the history of external civilization and of the special arts of Greece, yet no character in Greek religion has such wealth of ethical content, or counts so much for the development of moral ideas, as the character of Zeus. At times he seems to overshadow the separate growths of polytheism; and at times in expressing the nature of Zeus the religious utterance became monotheistic.
The study of this as of the other Hellenic cults must consist in great part of an examination of the cult-titles, which must be carefully distinguished from mere poetical appellatives, and which on the whole are our most direct evidence of the ideas embodied in the state-religion. And the importance of the title in the worship was of the greatest; for public prayer and sacrifice were never made to God in the abstract, but to a particular divinity usually designated by some term that showed what sort of help the worshipper needed and expected; unless he addressed the deity by the right title, the help might be withheld; and a great part of the function of the oracles in Greece was to instruct the worshipper to what deity under what particular name he should pray.