The Imperium of Man is a very devout empire. The God-Emperor is so titled for a reason, after all. Mankind's conquests are in his name. Prayers and sacrifices are sent to him by billions of worshippers. Entire worlds exist dedicated to faith.

But what of the larger Imperial Creed? For example, are the Primarchs worshipped at all? Can someone tell me about any major Saints or martyrs? What about religious practices?

We see a lot of the warrior side of the faith, but not much of proper observances.

Plushy said:

It's a difficult thing to define, simply because of the sheer scale of the Imperium. Matters will inevitably vary from one world to another, and from one sector to another, and Missionaries are skilled at twisting native beliefs into ones centred on the God-Emperor, meaning that there are (at least) hundreds of thousands of distinct variants of the Imperial Creed, and these are in continual flux, with variations deemed as having strayed too far often being eliminated by the Adepta Sororitas (in their role as the internal police of the Church - the largest-scale of these actions are regarded as "Wars of Faith"), while new variations spring up all the time and are spread by pilgrims and colonists, merging with local traditions and creating yet more variations.

Within that, the Primarchs are likely to receive worship alongside the Emperor, as His Sons, particularly on worlds made compliant during the Great Crusades, and especially on worlds that the Primarchs actually set foot upon (the stasis-locked body of Roboute Guilliman on Macragge is a site of pilgrimage for all of Ultramar and beyond, for example). Veneration of the Primarchs logically pre-dates organised religious worship of the Emperor (though disorganised worship existed long before the Adeptus Ministorum was founded), simply due to their connection to the Emperor, their deeds and the sheer force of presence they possess (it's been noted that some of the Primarchs' had such force of personality that mortals could not easily bear their presence - itself a lesser form of the Emperor's radiance). However, this only extends to the nine known loyal Primarchs - you wouldn't find veneration extended to Horus, Fulgrim or Peturabo on an Imperial World, at least not legitimately, and it's easy to imagine that many of their laudable exploits have been excised from the history books or re-attributed to others in light of the Great Betrayal.

Saints are another matter. There are potentially billions or even trillions of individuals venerated as saintly across the Imperium, and most of those are regarded only locally (upon a single world or across a single sector). Only very few individuals are significant enough on a galactic scale to be regarded so highly across the entire Imperium - the first that springs to mind is Sebastian Thor, whose actions contributed highly to the end of Vandire's Reign of Blood and who reformed the Ecclesiarchy in the years afterwards.

Martyrs would be even more common - the Imperium is oft-noted to regard martyrdom as a virtue, as the idea of giving oneself utterly and irrevocably in defence of faith and species is one that the Imperium is dependent upon. Martyrs would be possibly too abundant to acknowledge all of them, though I imagine that convents of the Adepta Sororitas keep a record of all of their Sisters who are Martyred in the course of their duties.

Religious practices… well, there are countless distinct cults of worship, focussed on differing aspects of the Emperor, collectively ruled by the Cardinals who oversee each Diocese (typically a planet, but some heavily-populated or significant worlds may have several - Terra is said to have hundreds), who in turn are collectively overseen by more senior Cardinals (typically covering a sub-sector equivalent area), then an Arch-Cardinal (typically covering a sector), and layers of hierarchy and politics between there and the Ecclesiarch (who commands the entire Adeptus Ministorum). With that sheer scale in mind - there are well over a million Cardinals in the Imperium, and each is a Peer of the Imperium, able to wield political influence akin to that held by planetary governors, Rogue Traders, Inquisitors and Astartes Chapter Masters - it should become clear that it's nigh-impossible to catalogue the religious practices of the entire Ecclesiarchy.

Imagine at the Cult of the Emperor (in whatever form is currently or locally prevalent) is equivalent to Christianity in this sense (and making no other comparisons for the sake of avoiding real-world debate) - a core and singular focus of worshjp, from which many different variations have sprung, both legitimate and otherwise. The difference here is, of course, scale - the Imperium covers approximately a million worlds and vast numbers of people with a colossal variety of conditions and origins, so it can be expected that any given Sector will contain anywhere from dozens to thousands of distinct variations of religion.

In one sense, this can lead to an overwhelming amount of scope and scale, but on the other it does permit immense narrative freedoms when it comes to defining what the Imperial Creed actually is. In either case, it's better to pick a starting point (a singular world, perhaps, or a collection of worlds founded during a single Crusade) and expand from there.

I think that you can probably find most of what you're looking for in the Blood of Martyr and the Inquisitor's Handbook books, as they would give a fairly good picture about the Imperial Creed.

But then again I'm not sure what you mean with "larger Imperial Creed" as it the whole fabric of the Imperial Creed is intervowen with the worship of the God-Emperor, but most can probably be made up with some heavy inspiration from the Medieval Catholic Church of other churches, sects and heresies.

I don't know if that gives an answer but maybe its something.

EDITED: N0-1_H3r3 got there first.

and don't forget you also have the Death Cults like us Moritats who kill to worship in his name

 Perhaps this seems obvious but have you read the core rulebook? It lists several saints and their likely followers. And in the Calixus sector, Saint Drusus and to a lesser extent Angevin are also worshipped. 

ranoncles said:

I would say saints are 'revered' or 'venerated', rather than 'worshipped'.

Adeptus-B said:

I would agree since I'm pretty sure that worship of any other than the God-Emperor is considered heretical and anathema to the Imperium. The only other faith that is tolerated is the Adeptus Mechanicus. And I'm pretty sure that many within the ecclesiarchy would get rid of the machine cult if they could get away with it.

Well, you could argue its a very fine line between venerating and worshipping. In real life, some people worship Maria and Jesus as much or perhaps even more obviously than they do God.

Still, the point stands that there are many saints (with a particular portfolio) and people in the Imperium probably ´connect´ more with a particular saint or slant of the Imperial Creed than with the God-Emperor.

ranoncles said:

I've never heard of anyone worshipping Maria, but I live in a very secular country. But Jesus, God and the holy spirit are one and the same, merely different aspects. At least that's what I've been told christians believe in, so they're just worshipping aspects of a god.

They might come close to worshipping great saints, the various sons of The Emperor if they were brought to compliance during the crusades but they'll never call them gods or venerate them as equals to the emperor. At least not if they wish to be in compliance with the imperial creed.

ranoncles said:

Much as people of the world today revere and make pilgrimages to the house of Mohandas Ghandi.  Each world probably has a unique variant on the Imperial Creed, so it makes sense that each one could have unique mythical/historical icons that are venerated.

EDIT:  It wouldn't shock me if a certain Primarch was venerated within Ultramar, yet studied as a great historical figure upon Tannith.

You all forgot the Cult Mechanicus

ranoncles said:

Best I can tell, that is actually the point of saints within the context of the Imperial Creed - individuals who the general populace can identify with, who serve as examples of the power of faith. As much as anything else, they're aspirational figures, and as mortal men and women uplifted by their faith they are more able to comprehend the trials of the common man and better placed to intercede in mortal affairs and convey the Emperor's benevolence on His behalf. Different saints embody different virtues and appeal to different sections of the population, and thus ensure that the Emperor's Light is spread as far as possible.

mrady said:

I mentioned the Adeptus Mechanicus earlier and it's technically a heretical, albeit, tolerated faith due to the simple unavoidable fact that without it, the Imperium could not exist. That is the sole reason why it is tolerated.

Primateus said:

my bad missed it

 I seem to remember that dating from 2nd edition 40K (1992-1996), Sanguinius was once stated as celebrated on an Imperium-wide scale, that his battle against Horus was a vital part of the Emperor's work, and that outside Macragge he is considered the greatest of the Primarchs by the common Imperial citizen. Not sure if this hasn't been retconned, though.

Sanguinius has his own day of celebration/worship in the imperium,  ill try to track it down