Mysteries in the ancient world were secret cults, requiring initiation ceremonies for those wishing to participate. Among those honoured with mysteries were Isis and Cybele, but by far the best-known were the Eleusinian Mysteries, centred on Eleusis in Attica, and devoted to the worship of demeter and dionysus (here the son of Zeus and Demeter, not Semele). Most Athenian citizens were initiated at some time in their lives.
The rites were agrarian in origin, like the thesmophoria. Presumably in archaic times they were supervised by the king, since in the classical era this function was performed by the archon Basileus. Initiation, which in early times was confined to Athenians, and then only with the consent of the head of the family, was later made open, first to all Greeks, then to Romans also. Murderers and barbarians were at all times excluded. The high reputation of the Mysteries is attested by the fact that the Roman emperors Hadrian and Marcus Aurelius were initiated. They were finally abolished by Theodosius at the end of the 4th century AD.
A certain amount is known about the Mysteries, but the details of the most sacred rites were always kept secret. There were the Lesser Mysteries in the spring, and the Greater Mysteries in the autumn. Taking the Homeric hymn to Demeter as a guide, we may assume" that the Mysteries were concerned with the Persephone myth, that Persephone's descent to the underworld was represented in the Greater Mysteries (i.e. the storing of the seed corn after harvest), and her return to her mother (i.e. the spring sowing) in the Lesser Mysteries. We know that the Greater Mysteries went on for nine days, and that after five days of preliminaries, there was a great procession on the sixth in honour of lacchus (son of Demeter, later identified with Dionysus) along the Sacred Way from Athens to Eleusis. Of the rites thereafter, we know only that they contained 'things spoken, things seen, and things performed', and that full initiates were called 'seers'.
The Mysteries are of great interest in the evidence they provide of the views of the ancient world on an afterlife. The Homeric hymn promised to initiate wealth and a happy life after death (i.e. continued celebration of the Mysteries, according to Aristophanes). Moreover, they show a marked advance in moral ideas, in that gradually ritual purity came to be seen as insufficient for initiation without goodness and gentleness also. This is perhaps the nearest approach in the pagan world to the Christian concept of sin.