+++ Any person who speaks ill of the Emperor, the Imperium, cites his loyalty to any entity besides the Emperor, defaces holy artifacts or buildings, incites heretical thoughts or actions, talks openly about forbidden subjects and generally behaves in a manner disrespectful to all that is holy and good will have his extremities removed and left to bleed to death, for the Emperor's pleasure. The body will then be burned to ensure no taint remains. +++ The Imperial Infantryman's Uplifting Primer, Art 6741/09a +++
must be regarded with the appropriate seriousness
In the most basic WH40K sense, heresy (or HERESY!) is the most severe accusation the Imperium could accuse you of which is mostly punishable by death. Heresy is mostly willingly consorting with Chaos, which the Imperium gravely forbids and anyone guilty of merely saying anything good about Chaos or American pop music will merit a fist-fuck of Inquisitorial proportions when they least expect it. The other popular form of heresy is the blasphemy and defamation of the Emperor and the Imperium, which is basically saying anything remotely bad about the Emperor or the Imperium. The others are basically anything against the Imperial Creed which is pretty much everything not related to the veneration of the Empra.
The trials for heresy are broad, but the majority of them occur like this, or this, or even this.
 What IS heresy?
The Imperium realizes that Heresy is a serious matter that must be explained with great detail from the lowliest servant to the highest Commander. And so with the aid of the most wise Inquisition and holy Ecclasiarchy, they have defined what is heresy in the eyes of our immortal God-Emperor:
- EVERYTHING IS HERESY
If you are not sure if something is heresy, it probably is. Play it safe and report it so that it can be properly blammed.
 But Seriously, What Is It?
Extra heretic Henry IV re-evaluates some of his life choices.
Well, outside of the warhams heresy was a word used to describe deviation from established religious doctrine. It's usually used by the Catholic Inquisition, who went absolutely apeshit for it in the Middle Ages. Being as how he held the keys to the kingdom the pope could issues orders of excommunication and interdiction, meaning the affected people and/or countries could no longer partake in the sacraments. So you weren't allowed to eat their juice and crackers or go to their clubhouse, who gives a shit? Well everybody. For starters it was a one way ticket to hell, which people took more seriously then than they do now. Also there was the little issue that all important oaths of the day were sworn before God, meaning excommunication rendered them null and void, and you were essentially an outlaw. Your knights didn't have to serve you, your peasants didn't have to pay their taxes, and if one of them tortured and killed you they got a high five from Jesus. Since the church functioned as the court of last appeal for most major matters this gave them incredible power, which they wielded like a club to bludgeon intransigent monarchs into acting right. That isn't to say there wasn't real heresy going on. In a society as insular and overwhelmingly illiterate as medieval Europe people developed some funny ideas about Christianity. Heresies ranged from the fairly pedantic (Christ was pierced with three nails, not four!) to the pants on head crazy (Catholic mass is a satanic ritual to shackle Jesus' spirit to his dead body so he can't save the world!).
Like all catchy memes it was eventually smote into the ground through overuse. Popes regularly threatened excommunication to kings, bishops, witches, countries, particularly retarded crusades, the city of Jerusalem, each other, and generally anyone who wouldn't do what the Church wanted them to do. By the time the Protestant Reformation (which was naturally labeled heresy itself) started to gain ground, excommunication was just seen as the price of doing any sort of business with/against/anywhere near the church. In the present day, most forms of what would once be called heresies are simply ignored because they ruined their reputation among Protestants, atheists, the scientifically-minded and sane people in civilization forever by declaring Galileo a heretic for writing about the scientific fact that the Earth orbited around the sun. Ever since that time, the Catholic Church uses the stigma-filled term "heresy" itself sparingly lest they themselves be burned by Protestants and atheists for stupidity, forcing them to opt for other euphemisms such as "heterodoxy".
 What do they do with heretics?
If your heresy was serious but you are repentant, the church may strap you to a horrific war machine called the Penitent Engine, which is a bit like a Space Marine dreadnought except it's designed to be really painful and humiliating for the pilot. What's creepy is that it's an entirely voluntary way of seeking absolution (obviously, since strapping someone to a war machine against his will is NOT SMART). Considering how many of these are stomping around, it's a wonder the Mechanicus hasn't designed a not-insane version to be used by the Imperial Guard as an assault walker. ... right.
The Imperial Infantryman's Uplifting Primer says that a heretical guardsman should have his extremities severed and left to bleed to death. At the discretion of the commanders, he may just get moved to a penal battalion or classically BLAM'd.
On remote Imperial worlds, minor heresies naturally spring up all the time because the Ecclesiarchy has a weak presence, so visiting preachers may try to take a softer approach with these things. They may even tolerate some fanciful unorthodox beliefs as long as they don't offend the core values of the church.
Furries, the most common source of extra heresy
Extra heresy is a decree recently enacted by local Commissar Fuklaw. While we're still a bit sketchy on the details, it is clearly mentioned that Xenos love is Extra heretical (that just makes it even more appealing) but it is a well known fact that anything remotely related to Jersey Shore, Justin Bieber, Miley Cyrus or anything Western pop music is extra extra heretical, and so is furfaggatory.....weeelll unless you're one of the Space Wolves, anyway. After all, they were just badass space Vikings until a certain update and string of model releases (We're looking at you thunderwolves and Canis). Punishment is still the generic execution by your local commissar... those dic*BLAM* -Most virtuous and excellent officers of the Emperor. Actually, if the Emperor saw all this shit, he'd have a single manly teardrop from his eye.
 What isn't heretical
The Imperium's patented heresy detector.
Much like the whole Rule 34 and Rule 35 gig, there are a few things that aren't heresy. Here's a short and general list:
- The immortal God-Emperor of man
- Veneration of the immortal Emperor
- Dying for the Emperor in the most manly way possible.
- Brutally slaughtering and burning heaps of xenos, mutants, heretics and most especially Beliebers, for the Emperor. (If you can live long enough to fire that flashlight that is--)
- Discovering new planets so the Emperor can have even more wars. Unsullied planets + war = More burning Heretics for the Emperor, hurrah!
- Building gigantic Gothic cathedrals the size of sky scrapers for the Emperor.
- Rendering said cathedrals spaceworthy.
- Praising of the Emperor.
- Hailing to the High Lords of Terra/Administratum/Ecclesiarchy- Er, Emperor. Harder this time.
- accusing random people ofHERESY!!!
- Why are you still reading this instead of venerating the motherfucking Emperor?! You must be a heretic for having doubt! Die heretic scum! *BLAM*
The Official Imperial Stamp O' Heresy
In the 41st millenium, everything is absolutely heretical.
Half-Xenos cosplaying commissars? EXTRA HERETICAL.
When viewing this image if the first thing that comes to your mind isn't target practice! or heresy!, then please report to your nearest morale officer for execution.
Should not want... *BLAM* Suffer not the furry to live.
Heresy at it's finest..in a horrifying and heretical way, of course.
 See Also