Plague of Unbelief
The Plague of Unbelief is a minor event in Warhammer 40,000 towards the end of the Age of Apostasy, during the few years between the Reign of Blood and the Thorian Reformation. A rogue Cardinal named Bucharis, jealous over his peers' greater achievements, decided to build the biggest temple EVAR. To accomplish this, he enslaved the entire population of his home planet, Gathalamor. Apparently, this wasn't enough, so he invaded and enslaved the neighboring planets too.
As is their wont, a Warp storm decided this would be a great time to appear, coincidentally hiding Bucharis' empire from the Imperium at large. Because travel was all but impossible, Bucharis could suppress knowledge of the Thorian Reformation and let the enslaved masses believe Terra was a corrupt mess led by a bloodthirsty tyrant. As such, Bucharis claimed, Terra was a lost cause and humanity should start over, centered on Gathalamor (although, apparently, he wasn't smart enough to think of what would happen to him after the Warpstorms abated). From there, he would conquer the rest of the galaxy, including, eventually, Terra itself. This did not work, because he tried to invade Fenris... yes, really. He tried to invade the home of the Space Wolves with Guardsmen and a few religious fanatics. Needless to say, the Wolves were not amused.
Meanwhile, trouble was brewing back at home. The newly-occupied world of Chiros was falling into chaos (no, not that kind), thanks to its valiant woodsmen. Eventually, after suicide bombings on commanders' tents, the army lost its backbone and fled.
And then another planet rebelled.
On the agricultural planet of Colcha, Bucharis' navy almost completely destroyed a rebel fleet, except one shuttle. This shuttle landed on Colcha, and Bucharis' Guardsmen waited.
A few months later, the (quiet, peaceful) planet went berserk and murderized most all of the Guardsmen. With this army depleted, Bucharis found he was having trouble keeping his coffers filled. And that's when a mysterious messenger came to Bucharis' palace and declared himself the emissary of Dolan Chirosius, later to be known as the Great Confessor.
Dolan Chirosius is basically the most potent Confessor of all time. All those rebellions and revolutions? Yep, Dolan's oratory caused all of them. This, understandably, pissed off Bucharis, who ordered him found and put on trial (after all, no need giving the people a martyr). Dolan gave a massive, hours-long speech about all of the rebellious actions he had done and how Bucharis was a despicable tyrant. Bucharis basically went "you're guilty as shit" Idiocracy-style and threw him into the dungeons for a few months. His corpse was all-but-unrecognizable.
This backfired spectacularly. The people, angered that Bucharis killed Dolan, finally began a rebellion on Gathalamor. Old men, women, and children faced down guns with their bare hands, and won, such was their fury. They stormed the walls and smashed upon the gates. Bucharis ran for a shuttle, but the great mass of the faithful threw themselves at the electric fence until its generators short-circuited. Bucharis did not get to the choppa, and was torn apart by the mobs bare hands. Not one single bit of the mad Cardinal was left to identify.
The Other Plague of Unbelief
In the now semi-sorta-retconned 13th Black Crusade, there was another, very different Plague of Unbelief. This one was an actual plague that killed people and turned them into Plague Zombies. This, understandably, made people very upset. When a guy calling himself the Voice of the Emperor declared that this was all the Ecclesiarchy's fault, the great mass of faithful once again listened and started rebelling. And then the 13th Black Crusade started for real.
The Voice of the Emperor may or may not be
Cypher A VILE TRAITOR. Either way, the Dark Angels suspect that he has connections to the Fallen OTHER VILE TRAITORS OF LESS LOYAL CHAPTERS WHO CERTAINLY INCLUDE NO SONS OF THE LION. Although they think that about everybody.
It also shows up in a Commissar Cain short story, which has the good Commissar fighting plague zombies. Its actually pretty good.
The Other OTHER Plague of Unbelief
Holy geeze guys...
The Black Dragons once had to cleanse a planet of more Plague Zombies, but instead of being slow, shaky, covered in flies, and hungry for flesh, these zombies were loud, as fast as a riot of normal humans, didn't eat flesh, and by all accounts looked alive except for the fact that their hearts weren't beating. Turned out to be a sentient disease created by Nurgle. Known as the Doubtworm, it had infected the planet with a memetic virus consisting of the words "the Emperor isn't real", which made all the people go berserk and eventually merge into a hideous worm-like monstrosity. The "zombies" were the early stage of the infection, and attacked the "survivors" (actually victims of the infection's later phases) in a last-ditch attempt to save themselves and the "survivors" from Nurgle's clutches.
Yeah, a literal Plague of Unbelief this time; eat your heart out, Richard Dawkins. Space Marines and Sisters of Battle are immune because they know for a fact that the Emperor is real (the former from their connection to him via the gene-seed, the latter because the victim needs to have some small amount of doubt in the Emperor to be infected). Also, the disease was smart enough to leave the planetary governor's family in their living human forms after it infected them, so they could pull a Genestealer and be evacuated off-world to infect more people. Thankfully a massive badass named Sister Sethano was present and convinced the Black Dragons to cyclonic torpedo the Doubtworm's ugly wormy face.