From the introduction:
The setting is a church not solely a physical building, although buildings and architecture are important, but the church as an institution , a significant feature of society. The events of play concern people who hold official positions in its hierarchy, at any level and in any capacity, and who live and work primarily, not necessarily exclusively, on church property.
The religion in question should feature one or more extremely central personal figures. The most obvious is the Abrahamic tradition, but as far as I can tell, nearly any major world religion and any number of historical ones can lend their concepts and images to this construct. Remember that it can begin, and even stay, extremely sketchy.