Mercerism was a religion followed by the denizens of post-World War Terminus Earth.
Mercerism centered around a martyr known as Wilbur Mercer. Together, followers would fuse with Mercer through the use of an empathy box, during which they would see Mercer endlessly climbing up a hill whilst he was endlessly pelted by stones.
In actuality, Mercerism was manufactured by an unknown party; Mercer's ascent was filmed on a soundstage. Mercer himself was played by Al Jarry, while the rocks hurled at him and injuries sustained were fake.