The Children of the Cathedral (commonly abbreviated CoC) were a widespread religious cult that the Master established as a cover for his activities and to spread his philosophy. Publicly, the Children were a post-doomsday cult that preached peace and unity. They worshiped the Holy Flame that destroyed all the world's evil (the nuclear fire that destroyed civilization in the Great War) and talked of their Master who would lead them to a better existence.
But they were more than a creepy but altruistic cult. The entire religion was elaborated by the Master to cultivate a pool of normal human operatives in the wastes and to prepare the world for his coming. Every person the CoC converted to their faith was one more person who could willingly be dipped in FEV. All Children eagerly awaited their chance to be baptized in the great baptismal font to the north - the vats of FEV in the Military Base.