Mystery religions were like secret clubs that people joined a long, long time ago.
Imagine being in a clubhouse with your best friends, where you all have to keep everything secret and can't tell anyone else what you do inside. That's kind of what mystery religions were like, but instead of just playing games and hanging out, they were about worshipping gods and learning special things about them.
People who were in the mystery religions believed that they had special secrets that no one else knew. They thought that by keeping these secrets secret, they could get closer to the gods they worshipped. They also believed that the gods would give them special things like good luck, happiness, and maybe even eternal life if they did everything right.
They would often hold special ceremonies where only members of the religion were allowed to be present. These ceremonies were really important and everyone had to take them very seriously, which made them even more special.
The mystery religions were really popular a long time ago, but they aren't around anymore nowadays. People don't really know everything that the people in these religions used to believe because they kept everything so secret, even from people who weren't in the club.