Mystery plays were based on stories in the Bible, the lives of saints, or on a Christian virtue such as Charity. They were performed in the open air in larger towns during May and June by a small company of about six players who were not professional actors but members of guilds.
The actors would typically be on an improvised stage with wheels such as a decorated farm cart.
Each guild would perform a different story and try to outdo rival guilds, so there could be dozens of mini plays being performed in different parts of a city on the same day. Each company of players would move from place to place on their wheeled stage, repeating their play to a different audience. Spectators might follow a company that they knew, to support them.
Mystery plays fell out of favour with the Church and were discontinued because they became too bawdy to be associated with religion. In the twentieth century they were revived in a few places with a long connection to the medieval plays such as York and Chester.
You can read more and see some scenes from modern performances of mystery plays here.
There is an archive of York Mystery plays here.
Here is a crash course in Mystery Plays (under 12 minutes).
Write your own mystery play based on a story from a religious book and include references to current affairs with a moral theme.
Write for a cast of about six principal players.
Devise some simple costumes for your characters with a distinctive headdress to identify each character.
Record your performance for posterity.