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 first tried to invade Fenris 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 (or what was left of it after the Space Wolves were done) destroyed a rebel fleet except for 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 to be tortured 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 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 mob's bare hands. Not one single bit of the mad Cardinal was left to identify.
This would normally have been the end of it but Bucharis' actions had caused such a resonance in the warp that it became an incredibly potent warp presence. During the opening stages of the Indomnitus Crusade, Gathalamor was attacked by Chaos forces and Bucharis's hidden tomb was found. Using his signet ring as a focus, the chaos forces were able to construct a device that somehow twisted the echo of the warped faith of "Bucharis Gift" into a weapon that could destroy whole fleets. Gathalamor was retaken but not before the important parts of the weapon were taken away.
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. 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. Really makes you wonder how much people actually trust the Ecclesiarchy. Fanatically worshipful of the Emperor? Yes. Trusting the Ecclesiarchy’s claims to speak for him? Maybe not so much after all.
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.
This plague also shows up in a Commissar Cain short story, which has the good Commissar fighting plague zombies. It's 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, shambling, 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. It 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 phrase "the Emperor isn't real", which made people go berserk and eventually merge into a hideous worm-like monstrosity. Yeah, a literal Plague of Unbelief this time; eat your heart out, Richard Dawkins. 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; in other words the zombies were the good guys trying to save everyone's souls. 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 offworld to infect more people.
Thankfully Space Marines and Sisters of Battle were 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 by sheer strength of faith). A massive badass named Sister Sethano was present and convinced the Black Dragons to cyclonic torpedo the Doubtworm's ugly wormy face.