Horrorism is a general belief in horrors, and is what makes horrors exist.
Horrors are believed to be surreal, ethereal entities, nondescript in appearance but only known to be horrific, whose very sight might inspire madness in the onlooker. They are believed to come from another plane, the Void, which can enter the real world (the world of Faith). Horrors are understood to be entirely alien-like, and it is impossible to understand or communicate with them.
Horrors are also attracted to "marks" called horrormarks. Once someone has been "marked", they are cursed for life, as it is believed that there is no way to remove them, and so the person will be more likely to be visited by horrors in the future.
There are generally understood to be two kinds of horrors: false horrors and true horrors. False horrors are lacking in power and can be defeated, for they are not true horrors (by definition). (True) Horrors are understood to be nigh impervious to spells, and to perceive dimensions differently from humans so that even spells directed at their surroundings (such as imprisoning force fields) have no effect on them. Some are believed to emanate anti-spell fields that can strip nearby spellworkers of any protection (such as force shields, repelling, invisibility, and notice-me-not spells) and prevent spellworkers from doing anything to escape (such as teleportation). By definition, then, true horrors are horrors that one cannot defeat, whereas false horrors are ones that one can defeat. Hence, defeating a horror means that horror was a false horror, and not nearly as dangerous as a true horror; hence, defeating a horror does not mean that one can defeat true horrors, and so defeating horrors provides no confidence for the spellworker targeted by horrors in general.
Horrorism was designed as a counter to the threat of certain overpowered forms of spellworking, and so it is a manufacted belief. Each of its forms counters a particular threat specifically, and does so quite well, for the belief was designed to expose weaknesses in those particular forms of spellworking. It threatens retribution on those who would work such spells, and thus is classified as a retributive belief.