Church
From the Slaves Wiki
The Church is a clan headquarters. Stand at it and press E to found a clan, run its members and declare war. It is the most expensive building in the game and you can only have one.
Workers
Nobody works at a church.
Clan altar
From the altar you found a clan (free), invite up to 10 members in total, and declare war on another clan for 500g. See Clans.
Building it
Open the build menu with B, pick the Church and place it within 200px of yourself. Buildings need 80px of space between them. You can own up to 1.
Demolishing returns 50% of the materials, and anything stored inside comes back to your inventory.