Abominationism is a belief in abominations, which is what makes abominations exist.
Abominations are horrifically deformed or twisted beings. They are believed to be created when a spellworker attempts to create new sentient life - though this only happens some of the time, and is believed to happen more frequently for spellworkers who are doing this for the first time as they are believed to not know how to do it “correctly”. Abominations thus resemble in some uncanny way whatever being the spellworker was attempting to create, being made of the same material and the same amount thereof, but in a different way. The abomination is most likely hostile to anything and everything near it, including its creator, and will proceed to use its sentience to work malevolent spells (assuming its creator endowed it with profound magical capability) in an attempt to kill its creator and then to put an end to its own suffering, but until then, it is a screaming, tormented wretch that cannot be communicated with.
Many times in the history of the world, spellworkers discovered that if they could create a sentient lifeform that was enslaved to their will and yet which believed that it had profound spellworking ability (such as believing it could shape reality to its will), then they would have power beyond compare. This is a similar workaround to the memory alteration spells that led to the Memory Revolution: “reality hacks” with the power to reshape the world.
Abominationism was designed as a response to that threat, so that a good percentage of such spellworkers would discover that they had summoned a powerful being that’s out to kill them instead of submit to them. The same powers of spellworking that these creators would give their minions, these abominations would use against them. However, the belief was designed by a circle of those who had already created minions in this way, who were jealous of their power; they created horrorism to deny others from following in their footsteps, hence the aspect that these abominations almost always happened only to those who were new to the art of creating new sentient minds, and not to the existing practitioners.
The creators of abominationism used their power to successfully spread it far and wide, preventing others from working the kinds of spells they worked. But in time, the old practitioners died out, leaving behind a belief that would wipe out many new practitioners but not checking the power of those among them who succeed to become adepts. Since then, the creation of new sentient minds has become forbidden magic as a result of this belief.