That is exactly the heart of the matter. In ArM5 both the mentioned relics and the True Faith of the Cathars would work. I am trying to sort this out without providing the players with a "truth". Once you make one side infernal or at least proven wrong by a simple check it is too easy for the players to take sides.
And that is not a small matter...
Why that?
Cheers
I want to thank the people, that posted in this thread for a very interesting discussion. My knowledge of medieval religion is much lesser than yours and so I contributed only one post, but read the debate with great interest.
I assume, you all know the essential passage, that discusses your problem in "Realms of Power: The Infernal", but because no one referred to it, let me quote it:
(RoP:I, p. 133-134)
For me the most important word is not "one God", it is "selflessly".
Maybe in rare cases a storyguide has to decide about the nature of an "evil act" (because nowhere you get explained in detail, what is "evil"), but generally I have no problems with this topic: If you have a relic with divine power and you don´t believe in it, you´re not forced to use it. If you are in a divine aura of a place, that´s aligned to the religion of your enemies: Don´t be afraid, God is with you, too! If you are wondering about the effectivity of the "false" relic in the hand of others, God wants you to think about your own fundamentalism. If I´m reading about the "Dark Gnostics", I think: Yes, there may be corrupted cathars, but corrupted catholics are existing, too.
The bible contains many gospels, some of them are antithetic. If you´re searching for a passage, that authorizes a split between catholics and cathars, simply visit the other side, too: Read about Jesus, who told his disciples to turn the other cheek when hit. Read about the good Samaritan. Read about the camel that goes through the eye of a needle more easily than a rich man enters the kingdom of God. The catholic church may claim, that the pope is the agent of christ on earth and infallible, but Ars Magica claims, that this is not the exclusive truth. I think, there are passages in the bible that support a division of the divine realm and passages that contradict this. Ars Magica has made a decision and it is not a bad one in my eyes.
Chiarina.
The view that ArM presents here is interesting. It is possible for a Gnostic worshiping "God" to be an Infernalist even if they worship the Good God as opposed to his/her bad antagonist (I don't know a historic Gnotic tradition that actually worships the antagonist rather than the "true" god). It raises the question if you actually can be a Roman Christian, worship God attend mass etc and through your sinful act still be an Infernalist? I would say "no" to that. A true infernalist would loathe to attend mass and stay within the Dominion. I would also say that a Gnostic tradition that does not actively worship the antagonist/Satan/Saklas would not be Infernal. If that was true an Infernalist who preach that kind deeds have moral value would be Divine.
Not really. If a "Luciferian" tradition preached that their Lightbringer was the One True God and preached a code of ethics similar to the Seven Heavenly Virtues, and didn't draw upon the Infernal for their "holy" magic, then yes, it'd be a Divine religion. If the tradition worshipped the Lightbringer as the Adversary, that would be infernalism and not Divine, even if the religion were somehow otherwise virtuous (the religion is still inherently corrupted by its rejection of the Divine); plus, that "otherwise virtuous" part would die an ugly death once demons started meddling with said religion. Meanwhile, a virtuous tradition in the name of God that draws upon infernal magic is also sinful and infernalist in character (exempla gratia: the Mulhidun; their religious precepts are theoretically well-intentioned, but their use of the Ars Goetia and the enslavement of demons is still spiritually corrupting). And, of course, an unvirtuous creed is not Divine.
But we still have a problem here. The Dark Gnostics think they use Divine power and the hypothetical Good Luciferians think they use Infernal power. How is this solved with game mechanics?
They both use Divine power, and are each wrong about the others beliefs. In fact they both hate the infernal (which the luciferians call Jehovah) and love the divine (which the luciferians call lucifer). They're drawing power from the same source with different names.
If the Good Luciferians think that they're using "the true Divine power," and they're not being actively deceived by a demon and granted actual Infernal powers, then they may indeed have True Faith and be granted Divine power. If they know that they're calling on demonic power, then they're infernalists, and if they were deceived by demons into calling on a power that the demons pretended was divine, they're also infernalists. The Mulhidun, again, appear to be an example of the latter case; they've been horribly misguided by demons and their tradition is in fact a tool of the Devil. (In Mythic Europe, of course, it's extremely likely that any Mulhidun or similar Good Luciferian who would otherwise develop True Faith will receive angelic assistance and a chance to reject their false teachings. Few would accept, of course.)
The Dark Gnostics think that they're doing God's deeds, but they're deceived; for one thing, their actions are sinful, and for another, they use Infernal power. Same with the Malleus inquisitors; just because they torture and murder in the name of God doesn't mean that they're not actually Hell-bound.
So let me sum up;
This is one of the points that Screwtape makes to Wormwood in CS Lewis' "The Screwtape Letters" - that active evil tends to be less effective in bringing souls to Hell as passive, petty, mediocre evil. The big, bold actions make for better cartoon villains, but the soft, slow road to Hell is a much more effective path. Infernally speaking, of course.
Remember the big 50 might demon in the Core ars? Bartholomew? That guy wins souls to hell by convincing people to follow his methods for business. Which I might add are not ever explicitly sinful.
But yeah, I've always found it to be weird that infernal automatically makes it evil, even if you aren't aware the power is evil. I would say that infernal powers are like Bartholomew, always making it so easy to slip into sin without really noticing it. The instructions to use those infernal powers don't technically ever recommend committing sins, but everything would be so much easier if you committed a small sin or two.
And this is the version of the infernal that I like. I have no idea if it is in paradigm (I guess it is not, since the general impression is that ROP:I basically ditchred it in favour of cartoon demons, so I guess those are more in paradigm) but the scheming (victorian?) demon is funnier to play. 
Evil has selfish purposes. Evil has the cartoon and villainous and it has the subtle and seductive. All individual beings have their own motivations, and may come to cross purposes; infernal beings want to gain souls for Hell, how they do this is up to the individual, and is probably based on how powerful they are (might is reflective of this).
Bartholomew may want nothing to do with the cartoon type of beings, his methods have been perfected over centuries (he's a might 50 being, after all, he was either this way, or became this way slowly. I like the idea that he's become refined. He can almost fit into everyday life perfectly. He's the demon you don't see coming, while the carton ones are blasted away. He may even be involved in some of that blasting, because those demons get in his way. There is an unanswered question, the donations he's given the Church should almost certainly be causing problems within the Ars paradigm. Of course, that suits his purposes, too. But his money is mingled with other moneys, so it is never understood that he and his donations are the cause of the problems.
No, what the Infernal powers in Mythic Europe should do is stop war, help old ladies, reduce famine and counter any trials stated by The Divine. They should start a New Dark Gnostic church and be really good and kind to people. They don't have to currupt people at all... just make them used to the Infernal like people are using electric power today. That all it takes to waste their souls.
Considering that "the Internet is for porn", that prolly true. :mrgreen:
OTOH, war has rape and pillage. Why waste a perfectly good tool that naturally brings you souls?
Sorry, am I trolling? :mrgreen:
Well, they are infernal beings - basically powered by pure hatred and selfishness. I would imagine that they can't actually comprehend many of those sorts of things as being a valid approach to success. At least in game mechanics, they'd have to be spending Confidence points for some of those things, and others they simply can't do.
Which is how I like to approach those sorts of "subtle" demons. While they CAN, using the core rules, implement subtle and vicious strategies, it's not in the core demonic nature to do so. As such, they can't react to changes in their plans very well. So when the PC's come around, they've got a fighting chance of stopping the demon's plans (as it's got to spend a Confidence point every time someone messes up his carefully orchestrated scheme). As otherwise, the PC's would be screwed.
So when dealing with that kind of demon, the underlying game mechanic is all about forcing enough changes in the demon's plan, in as short a length of time as possible, that the demon runs out of Confidence points, and throws their hands up in the air and quits (and/or directly attacks) in a fit of pique.
It is indeed a bit harsh, that infernalists whose actions might be considered good (by the standards of moral philosophy) and lead to positive results for others are nonetheless condemned to Hell. Nonetheless, to knowingly draw upon infernal powers is a conscious rejection of and blasphemy against God.
There are advantages to the Malleus method; namely, that it's very good for corrupting good people through deception. The Malleus Inquisitors think that they're doing good, but their actions are instead sinful, and furthermore, it's not just the hammerer who's corrupted by his actions. He's raising a lynch mob to murder innocents in the name of God, after all, and the simple peasants who join in with the torches and pitchforks stain their souls just as badly as the Inquisitor himself. That's the thing with corrupting great men, their fall tends to bring others down with them.
As for "good infernalists," it'd sure be nice for ol' Scratch if demons could make sure of just that, wouldn't it? Unfortunately for him, Hell corrupts and destroys everything it touches. The fact is that, first off, most infernal powers are either more difficult to use for good, or just flat-out impossible; for example, using Debauchery requires the user to commit some kind of sin just to activate the power. Secondly (and this is more of a storytelling concern than one you can reduce to hard mechanics), using infernal power corrupts and taints you; even without the Maleficia only working if you're willing to hurt someone to do it, the fact is that you're talking to and living among demons. Where most people have a guardian angel to prick their conscience, you've got a shoulder devil who is whispering that that girl over there would really have a lot of fun if you used a little Psychomachia to get her to loosen up a bit and go to bed with you. (No, the demons are not able to avoid trying to corrupt someone into further wickedness once they have their soul. It's in their nature to corrupt and defile.)
Now, where it really gets dicey is with genuinely well-intentioned and misguided individuals. For example, the Mulhidun are completely mistaught; they believe that they have a right and obligation to command demons and don't realize that it's a sin. (Enais, their False God, is laughing his peacock tail off.)
EDIT: Someone who is genuinely unaware that their powers are Infernal in nature is still an infernalist, but they are in fact not committing sin. As the Mulhidun do not know that their powers, in fact, come from the Infernal Realm, it's quite possible (depending on your storyguide's interpretation of this rule) that they are not committing sin by bringing demons under their command through the Ars Goetia. Of course, the Mulhidun are still liable for their other sins, such as becoming lazy from using demons to plow their fields, and their success and difference from their neighbors also inspires envy and hatred in nearby Muslim tribes. Even if a few Mulhidun do slip the net, the tribe as a whole are a tool of evil.
Sounds a bit like cathars, doesn't it? But then... weren't they Divine? 
Sound's a bit like the crusaders... that do have divine protection! What is the bleeding difference?
It's a Divine Mystery! (Which is an intentionally unsatisfying answer. God in Ars Magica sees no reason to make his plans clear.)
But for one thing, Crusaders aren't summoning demons. While I'm sure this makes no sense, killing people who worship God in another way and taking their stuff is (sometimes) a holy deed, while killing demons to take their stuff is Infernal. Power source, again. (Remember that the Malleus was a demonically-originated letter.)
However, that aside, I would personally note that the Crusades are a mixed bag, especially the Christian crusades; while the First Crusade was indeed divinely-backed (or at least God didn't withdraw his backing from the Pope's command), there are now both angels and demons manipulating the whole thing, the Fourth Crusade was corrupted by mortal ambition,the Albigensian Crusades are about to decay into such blind fanaticism that they lose sight of God ("Kill them all; God will know his own."), and God alone knows what the Templars are up to.
I think that the problem lies with the religions. One Crusade can't be good if the other one is evil. It can't be solved unless you actually have One Truth. All the other versions are heretic. That idea might be too daring for the game creators though...