Thaumetic corruptionism is the belief and meme that attempting to work any positive (reality-altering) spells would bring a Taint into the spellworker or the deity or spirit invoked by the spellworker to work the spell. It also describes the resulting effect on spellworking. It is also believed that attempting to work negative (reality-preserving) spells would not cause this taint, because with negative spells nothing is actually being changed.
The Taint is believed to be introduced into a person the moment they try to work any spells at all, though at first the Taint is so small as to be unnoticeable. Over time the Taint grows within affected individuals, so that what is first negligible is noticeable and then it continually worsens to the point that it breaks them. However the process is believed to progress at different speeds for different people, taking anywhere from seconds to decades, and the taint can suddenly worsen (progress faster), and significantly, at any time.
It is believed that while some may use spells to remove some of the taint, it cannot truly be purged, and will just come back (much like metastasized cancer). Furthermore, any attempt to eradicate or limit the growth of the Taint will also taint the spellworker attempting this. As its progression cannot be prevented, only slowed, this means certain doom for any spellworker.
The Taint is believed to originally be introduced into the "plane of magic", from which all spellworking is believed to derive power (at least per this faith). All spells are believed to tap into that plane for the energy needed to power them, and when the spellworker works a spell, one is contaminated by an infinitesimally small amount of the Taint. The greater the scale of the spell being worked, the more of the Taint that comes in.
The Taint is believed to grow within that plane, so that over time the "plane of magic" becomes more and more corrupted, and later attempts to work magic on the same scale will introduce exponentially more of the Taint into the spellworker, until even the smallest spell will introduce so much Taint that it instantly breaks the spellworker's mind.
The effect of the Taint (when it 'breaks' the mind) reduces the afflicted into mindless, blathering idiots, devoid of personal will and practically devoid of sentience. As a result, a person with a mind broken by the corruption cannot work magic. Spirits and deities affected by the Taint become unable to work their magic, meaning their followers can no longer work magic through them, and must do so directly, which would contaminate them with the Taint.
Thaumetic corruptionism is a modified form of the belief of mana corruptionism, but has much more far-ranging consequences. With thaumetic corruption, pre-collected mana pools and spellworking via non-mana faiths offer no defense. Hence, thaumetic corruptionism is a virulent meme engineered to be an ideological superweapon, against which the only protection is to not work spells at all. It was intentionally created, thus it is a manufacted meme.
Thaumetic corruptionism was devised as a response to the twin threats of monetized mana (mana monetarism) and the particularly potent brand of monotheistic religions emergent at the time, both of which would have lead to power singularities (hegemony) were they to allowed to progress. The meme was created with the intention of causing all magic to be abandoned, thereby "leveling the playing field" for the survivors and future generations.
Resistance and Consequence
Originally, only a few individuals believe in the concept enough to be tainted by their very beliefs, and the Taint starts off hardly noticeable, in line with the meme's statement that originally the "plane of magic" is almost entirely pure. Over time more and more individuals are broken by the corruption, including healers attempting to purge them of the Taint, and circles of spellworkers attempting to purge the "plane of magic" itself of the Taint. Because the Taint is believed to take any amount of time to manifest, use of any technique or faith to prevent tainting cannot be proven to work, and hence cannot gain any traction against this meme. Over time more and more become unable to work magic, and the terror of losing one's mind is enough to terrify the rest into abandoning spellworking altogether.
The widespread adoption of the meme resulted in all the deities of the world "dying", or rather, fading, as their spellworking ceased. It crippled all the technology that relied on magic, bringing civilization to its knees. And with the death of the sun god, the sun went out, and all was as dark as night.