Mystery religions worshipped gods from far and wide, from Greece, Syria, Anatolia, Egypt and Persia, among others. Although there were so many gods, their worship was alike enough that we can class them all as "mystery religions".
Mystery religions were largely secret, requiring initiation into them, and secrecy from believers - meaning that our present knowledge of them is derived from veiled language from initiates, or from the actions of converts to Christianity who felt no need to keep the vows of secrecy they had sworn.
The word "mystery" (mysterion) is derived from the Greek word "myein" which means "to close". Originally, this word referred to the closing of the eyes or lips, and may have been used because of the secrecy a mystes (initiate) was supposed to hold. At an an initiation ceremony, secrets of the cult were revealed to the mystai (initiates) which they were never supposed to divulge, and, indeed, could face legal proceedings if they ever did. It may also refer to the enlightenment that participation in the mysteries brought - indeed initiates were compared to babies who first discover the world through the discovery of light. Many of the terms used in the mysteries are light-based, such as the highest initiate at Eleusis being called "epoptes" - beholder.
Many of the religions were of great age, some coming from the many agrarian festivals which celebrated fertility of the crops, which was an important consideration in areas dependent on farming. Many of the gods - such as Demeter and Dionysus - were gods of farming, of rebirth in spring, and of fertility. Dying and rising gods such as Osiris, Attis and Adonis also reflected the agricultural cycle of winter and spring, of death and rebirth. Even Mithras, who otherwise had little connection with planting, was depicted as causing the grain to grow through the death of the bull he slew.
Ancient religins often honoured a Mother Goddess who sometimes had a consort, often a sky god like Zeus. The earth's fertility was linked to a marriage between these gods. Many mystery religions concentrated on a Goddess - for example the Eleusinian mysteries concentrated on Demeter and her daughter Kore. In many mystery religions the Goddess was either the only god honoured or, as the pre-eminent partner in a heavenly marriage, such as with Isis and Osiris. Even in the worship of Dionysus (who is rather obviously male), he was sometimes portrayed as an androgynous or effeminate figure. There is an exception to this general rule, however, in the worship of Mithras. While his worship had a connection with the worship of the Great Mother, Mithraism was a male-only cult and was very "macho". Its membership included many soldiers and its ranks of initiation had military overtones - the first stage of initiation made on a Miles, that is, a soldier. Mithraism also included trials of strength and bravery not found in female-centred cults.
Most mystery religions employed the dying-and-rising god motif (as mentioned before) and somehow the initiates felt that they too lived in a new way - whether through a form of rebirth, or in the hope of an afterlife. In the Eleusinian and Egyptian mysteries symbolic death and resurrection was practised, and initiates into Mithraism were declared to have been reborn - in a remarkable resemblance to Christianity. Many mystery cults also had communal meals and other similarities to Christianity - which has itself been described as a mystery religion.