The mystery religions were not religions per se, but rather secret cults of the Greek and then Roman world. Their number included that of Dionysus, Demeter and Persephone, and Orpheus. A Greek citizen, and then essentially anybody once the Romans conquered Greece, could be initiated into the cult of their choice.

Many have gone as far as to suggest that Christianity borrowed or inherited much from the mystery religions. Fasting and confession of sin was sometimes done, and, some claim, even baptism. Furthermore, some have pointed out that the mystery cult of Dionysus has a story of Dionysus very similar to that of Jesus. The reply is usually that the ideas developed in parallel, and the fact that many religions all over the world have similar kind of things going on.

Though they spread like wildfire all over the known world once the Romans conquered the Greeks and Hellenism became all the rage, nobody today is really sure what was up with the mystery religions. The reason for that is this: the punishment for divulging the cult's secrets was death. Despite the hundreds of thousands of people initiated into the various cults, we still know little about them.