Whistling Beaver Brewery
A hostile outpost in the far east of Faith's region. The large central building prevents the cultists from coordinating effectively, so start by taking out the ones on the roof, then walking your fire around the building.
There is a region map here.
The safe is in the main brewing hall (the one with all the metal tanks). Go there, and face south. You'll notice the leftmost tank has a spce you can crouch through.
The end of this passage has the safe.