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Heresy in Warhammer 40k can be one of any number of things (it’s not hard to get accused of heresy) but it is best defined as believing in ideas that go against the Imperium Cult of the God Emperor of Mankind, as defined by the Ecclesiarchy.

But as much as it’s easy to say that there is no room for tolerance of heresy within the Imperial Cult–Inquisitors and Sisters of Battle exist to root out heresy–there are conflicts within the system, most notably the machine worship of the Adeptus Mechanicus, and also the individual cults of certain Space Marine chapters.

So what is heresy in Warhammer 40k? Who decides what counts as heresy, and what is done in the case of heresy?

Definitions of Heresy in Warhammer 40k

While any differing religious viewpoint from the Ecclesiarchy’s Imperial Cult can be heresy, simply having a single “wrong” idea is not often enough to be labeled a heretic. True heresy is when a person or group (heretical cults) choose to worship gods other than the God Emperor of Mankind, and especially when people choose to turn to the Chaos Gods (though believing in Xenos gods is bad as well, it’s just not as common).

Worshipping any god other than the Emperor is punishable by death, and extreme lengths are gone to to root out and exterminate heretics. (“Purge the witch! Burn the heretic!”)

But accusations of heresy in Warhammer 40k are so common as to be a two-edged sword, often glancing back at the accuser and brining the Inquisition or Adepta Sororitas down swiftly on the heads of both the accuser and the accused. It is this blanket policy of wiping out everyone tainted simply by making accusations that leads many lower-tier Imperial subjects look the other way when heresy is present, lest the Inquisition burn the whole place to the ground.

Exceptions to The Laws of Heresy in Warhammer 40k

There are two big exceptions when it comes to the definition of heresy in the Warhammer 40k world. The first is the Adeptus Mecahnicus machine worship, and the second is the Chapter cults of the Space Marines. There is also a third, semi-questionable group in the psykers.

Chapter Cults of the Space Marines

While every loyalist Space Marine is loyal to the Emperor, they may not necessarily believe that the Emperor is indeed a god. Rather, they often believe that he is simply the greatest man who has ever lived and who will ever live. They are strictly loyal, but they fall short of actual worship. This, of course, doesn’t fit in with the Imperial Creed that the Emperor is a god. But, seeing as how the Space Marines are a power unto themselves and not easily reigned in even by the most powerful Inquisitor, this is generally overlooked. Individuals with extremist views might be declared to be heretics, but it takes some really massive heresy for an entire chapter to get that treatment.

The Cult Mechanicus

The Cult Mechanicus is the religion of the Adeptus Mechanicus, as led by the tech-priests, and they worship their own god, the Machine God. There is a band-aid fix that is applied to this problem, however, and it is that the Ecclesiarchy has extended their rules of what can be judge non-heretical to include the idea that the Emperor of Mankind IS the Machine God, in a form known as the Omnissiah. The Omnissiah is the physical manifestation of the Machine God in the Materium.

This is not to say that there aren’t Cult Mechanicus tech-priests who take their worship of the Machine God further, worshipping the actual machines that they are working on and maintaining. These will sometimes be called out for heresy in Warhammer 40k, but not often.

The Adeptus Mechanicus is in a powerful place of privilege in the 41st Millennium, and that is because they are responsible for so much of the might and power of Imperium. It is the Adeptus Mechanicus who builds and maintains all the machines, weapons, vehicles, ships, and every other technological work that the Imperium uses, and they are therefore indispensable. For the Ecclesiarchy to come down harshly on the heads of the Adeptus Mechanicus would be to cripple the Imperium itself and make it weak and defeatable.

So, the Cult Mechanicus is allowed to remain, per the Treaty of Mars in the 30th Millennium.

Psykers

Psykers are often looked on with skepticism and distrust, sometimes rightly so. A lone psyker who is not part of any Imperium organization–a so-called “non-sanctioned psyker”–is a heretic, and can be put to death merely for having psychic abilities.

But there are problems with this, some of which are that Adeptus Astartes have sanctioned psykers working for them all the time in Librarians. But the second, and far more pressing issue, is that psykers are needed as Navigators. If it weren’t for psykers, Imperium ships would not be able to travel through the Warp as they do.

For that matter, whole Chapters of the Adeptus Astartes would be heretics without special consideration: the Grey Knights are all psykers, and they are viewed as among the best that the Space Marines have to offer.

What It All Boils Down To

The Imperium, as strict as it is, is a mess of contradictions and exceptions. In a world that is as teeming with different energies, magics, powers, and literal gods, daemons, and angels, it is going to be impossible for there to be hard-and-fast rules for what is and what isn’t heresy. Some of the best books in the Black Library, including Eisenhorn, reflect this as the Inquisitors lead lives that are shades of gray, working both inside and outside the laws, making judgements that only they as ultimate arbiters of good and evil can make. The decision that one Inquisitor might make could be very different from the decision that another Inquisitor would make in a similar situation.

Punishment for Heresy

The pursuit of heretics is the role of both the Inquisition’s Ordo Hereticus and the Adepta Sororitas. The Sororitas are the military arm of the Ecclesiarchy, and are sent to execute punishment, but they do not make the strategic decisions of who is a heretic and which world to attack. (They do, however, make battlefield decisions along these lines, exterminating some and sparing the lives of others.)

The decisions of who is deemed heretic falls on the Ordo Hereticus, who investigate seeds of suspicion. The investigation can last hours or decades, but once the subject has been deemed a heretic, the time for purging has come. Purging the heretic rarely spares bystanders, as nearly all who are touched by the taint of heresy will need to be destroyed. There are exceptions, times when the Adepta Sororitas are viewed as angels of mercy and not killers, but such times are rare.

Excommunicate Traitoris

The first step in punishments of heresy in Warhammer 40k is to term the deem the heretic as Excommunicate Traitoris. This is a banishment from the Imperium on pain of death (though the target of this Excommunicate is seldom given the chance to run. If the heretic is present, they will immediately be purged.)

When someone is declared Excommunicate Traitoris they are removed from Imperium records through the Deletion Teams of the Administratum’s Historical Revision Unit.

There will be standing order for any Imperium unit to exterminate the heretic wherever they are found, whether they are caught by the Inquisition or the Sisters of Battle or the Space Marines or the Imperial Guard. They are to be wiped out on sight. Consequently, most Excommunicate Traitoris flee to the help of those they consider their own gods, be they Xenos or Chaos.

Edict of Obliteration

There is a further step that can be, and is often, taken for heretics, which is an Edict of Obliteration, where they are scoured from Imperial History as though they had never existed. It is rumored that this may be what happened to one or both of the two missing Space Marine Legions.

Exterminatus

Only the highest ranks can order an Exterminatus–a Chapter Master, an Inquisitor, a Lord Commander of the Imperial Guard, or a Lord High Admiral–which is the destruction of an entire world, and they only do it in the most critical of situations. It is used when an entire world has fallen, usually to Chaos corruption but occasionally Xenos, and the planet is determined to be irredeemable.

Playing with Heresy in Warhammer 40k

There are many ways that you can play Warhammer 40k with the motivation of purging heretics. The first and most straightforward is to play as the Sisters of Battle, whose primary motivation is the destruction of heretics. There is much to be done narratively with the Adepta Sororitas to depict the fights against Chaos. If you’re looking to get into starting a Sisters of Battle army, we recommend this article here.

The second option is to build an army around the Inquisition. This is a little bit difficult at the moment as there is no official Codex for an Inquisition army, and there hasn’t been since 6th Edition, but Inquisitor models can be added to Imperium lists, and there are some interesting models including Eisenhorn, Karamazov, and Kyria Draxus.

And of course there is always the option for narrative battles (which I am a big fan of, but which get overlooked in favor of Matched Play.)

On the flip side, if you want to play the heretics, your options are wide open. The best place to start is, of course, with the Heretic Astartes. A nice complement of Chaos Cultists will serve you well while we’re waiting (for so long) for Traitor Guardsmen. If you can get a copy of Blackstone Fortress or the proper expansion, Cultists of the Abyss, there were some terrific Chaos guardsmen that are perfectly iconic and would spice up any heretic army.

Conclusion

There is a lot of delightful storytelling possibilities on the subject of heresy in Warhammer 40k, and whether you create an army around a leading Inquisitor, or create a Sisters of Battle army that is dedicated to purging the heretic, I’m sure you’ll find Chaos players who are more than happy to throw their Chaos Cultists at you. And if you’re embracing heresy, send that Heretic Astartes to kill Eisenhorn.

Heresy in Warhammer 40k is an integral part of the game, one of the things that makes grimdark so grimdark.

What’s wrong with a little heresy?