Mjoellnir said:

The Hunters Hunted ones used faith or weak forms of thaumaturgy, the Hunter the Reckoning uses something really weird that is left undefined, though some products (Time of Judgment) state it "can be" from Lucifer, other products (such as Walking Dead) vaguely imply its a tie-in with Exalted, back when they were going to have Exalted be the prehistory of the WoD.

And the Dark Ages had a "Shadow Inquisition" with powers that seem a mix of Hunter the Reckoning and True Faith.

For me 'faith' in 40k and the effects that it has are a generally a warp phenomonominomnomnominon.

Faith abilities come down to one of three things:

1) Genuine self-guided psyker abilities that are attributed to faith. Some psykers are just psychic, but have faith and consider their abilities to have come from the Emperor. Sometimes non-psykers can have a hunch, vision or minor bit of warpy-stuff going on that is attributed to faith, but it's not really.

2) Acts of almost super-human acclaim that are accomplished via the willpower attained from strong faith. There is nothing 'special' going on beyond extreme effort.

3) Actual 'miracles' which come about as either concious or unconcious efforts from the Emperor himself. His own abilities achieve the result. He is a god created -as other warp deities have been created - by the faith of mankind worshipping him over thousands of years. This gives him the godlike ability to influence events in a minor way, and his subconsious mind is more predesposed to fleetingly linger on those who are most devoted. Such abilities usually don't 'detect' as normal psyker abilities. However, they still *are*, it's just that they are not recognisable as such.

As to 'breaking' someone's faith; I consider that if someone's faith is genuinely broken that they cannot purposely use such abilities outlined in '3'. However, that's almost impossible to do, because we're all stupid sheep, and will happily ignore evidence or re-wire our belief system to incorporate and discard things that conflict with our close-held beliefs. One only has to look at religion today, or those who 'don't believe' in climate change to see that the human mind will jump through hoops and act in a utterly illogical manner in order to preserve beliefs. Even direct conflicting evidence can be discarded as a test of faith.

You could literally hand the faithful a signed letter from the E saying 'I am not a god', and it would be somehow justified by them. Only weak faith is hard to break. Strong faith (the kind that seems to grab the Emperor's attention and allows him to influence things, as a dying spasm of his power) is pretty much unbreakable.

We're very good at believing what we want to believe.

Siranui said:

<cue Life of Brian>

Still, I'd say it depends. The case discussed in this thread - the Denounced and Condemned advance - isn't exactly the character stopping to believe in the Emperor, it's more about losing confidence in oneself, or rather one's "worthiness" in His eyes. And here I can definitively see why Faith powers would stop working, as they did as per GW's rules for the Ephrael Stern miniature*:
The character will continue to hold on to the faith per se, even pursue the same goals and still fight the same enemy, but will lack the utter conviction that the Emperor is watching over him or her due to one's own alleged taint. You're a mutant now. A witch. For a Puritan, the thought that such creatures would be "heard" is almost heretical, if not impossible. Oh, you may still hope for it, pray for it, but ...

It's that tiny speck of doubt that separates the real fanatics from the normal sheep. In essence, the character will revert to a normal follower of the Creed, still loyal to the Church and its dogma, but not in possession of the Pure Faith talent.

*: though this could, of course, also be a result of the combination of experiences Stern had as opposed to "just" the condemnation

That entirely depends. You can shove proof that the 'faithful' is acting in direct defiance of scripture under people's noses, or indeed of a theory that they do not support, and many faithful will self-justify still. "I might be a mutant, but this is a trial the Emperor has inflicted, that I come to know how the mutant is hated, and in that isolation grow stronger in faith...<rant> He still loves me."

Ultimately, it takes a much braver, honest and logical person to say 'I've screwed up' and suffer from self-doubt than it does to continue to believe that you are still 'right'. Who wants to believe that they are 'wrong', after all? That the person or thing that they love has stopped loving them back? The irony of faith is that those who have the most of it are often FAR further removed from the key tenants of their faith and crucial dogma than more 'casual' followers.

Siranui said:

After all, another old truth about faith is: it stacks. The more likeminded people you have around you, the easier it gets to maintain your convictions, as everyone will strengthen each other's belief. Yet, when this group - your only family in which you have lived your entire life - condemns and exiles you, I can absolutely see the reasoning behind GW's perspective.

Just as an addendum (it seems the edit function for the previous post has expired) - I can perfectly see why a Psyker who has grown up differently may also think differently. As you said, it depends.

Siranui said:

I definitely like this approach. I think that if you can "fall" from Pure Faith, it should only be at 100 insanity or corruption, as the loss of Pure Faith could only be construed as a form of insanity or corruption in and of its own right. While loss of the validation of one's peers would be traumatic, it is probably no more traumatic than, say, exposure to the raw energies of the Warp or psychically contacting the Tyranid Hive Mind, and the corrupting or maddening effects of either can be annulled using the Pure Faith basic ability.

I touch on this with a vodoun like tolerati. The woman still believes in the emperor, but not through the dogma of the ecclesiarchy and clearly not through their point of view either. She was the odd ball of the group, most prone to radicalism (being scum and force to survive in a uncaring galaxy can do that too), and had a deep interest with xenos (orks and kroot) due to her ordo xeno background. Personally if it was not for her high social skills which the GM basicly said that I can spend a thousand five hundred xp to get unnatural fellowship x2 I think the group would not take her in. That she is highly skillful in other areas too.

Her reason for studying the kroot is because she felt closer to them in terms of adapting to change. Corrupted ecclesiarchy clerics almost ruin her hive city by stealing the money to fix damage pipes. The result is that lower levels of the hive got flooded with toxins that mutated people. She was the only one not mutated due to sheer luck in being in the upper levels to steal thrones. She had to adapt to dealing with mutants in such a young age. When the purges began she was rescued by a inquisitor who was doubting himself over what the clerics did. That and he thought the woman's strange faith powers were psyker powers. When it came clear that it was not she was adopted. So she had to deal with that change as well.

Another reason for the kroot is that they seem to share the same religion. Given it was a big mistake, but the tolerati could not really argue that there are some aspects that her faith has and what the kroot believe. Then again her vodoun like religion is almost street shamen like and the kroot are shamenistic. It just got her attention.

Orks is simply because for a race that is stupid they can sure turn junk into gold. Teleporters made of spare parts and trash that when built by humans would never had work. Guns that should had fallen apart... It just did not make any sense at all. Till she captured one and through questioning she got her answer which shocked her at first. The orks simply believe that their trash can work. Thus the radical (now inquisitor) tolerati started to investigate the true nature of faith...

I guess the point I am making is that I agreed with Lynata faith waaaaaaaaaaaagh theroy to the point that I made some tolerti faith powers.