We're looking at the time period during and after the 3rd century BCE, when a number of early "mystery religions" developed in the ancient Greek world. What made them "mysteries" was that one had to be initiated into the cult to know its symbols and rites. They also often dealth with themes of death and rebirth. Three of these mystery religions are worth noting.
1. The Eleusinian mysteries were related to the Greek goddesses Demeter and her daughter Persephone. The rituals were related to the change of the seasons, to the harvest and to cycles of the growth and decay of plants.
2. The Dionysian Mysteries were related to the god Dionysus. Rites involved use of wine, as well as music and dance, to produce mood-altering states, though the beliefs of these cults incorporated ideas of death, rebirth, and transcendence.
3. Orphic Mysteries related to the figure of Orpheus, associated with a legendary journey to the underworld. Unlike the Eleusinian mysteries, which revered the agricultural cycle, and the Dionysian mysteries which revered the sense of transcendence that coms with intoxication, the Orphic mysteries emphasized asceticism and self-denial to achieve transcendence.