Let's say you have a hermetic diabolist. Chthonic magic and goetic arts. How would you go about keeping the taint from being discovered, so as to not create problems for the rest of the covenant?
Well sacrifice a Virgin each full moon maybe ? It can help a lot !
How does that help?
I would start with the general CrVi guidelines. But you might be able to work with something similar to Shroud Magic (MuVi) or Masking the Odor of Magic (PeVi).
More seriously I think that a good Aegis may be sufficient. Right ?
Devil is subtle anyway so only a holy being may be able to detect it.
IIRC there's a demon specializing in concealing Infernal Taint.
Maybe take one as familiar?
The problem is mainly with the mages, since catholics consider you an unholy witch anyway. For example, if you use effects of Chthonic magic or maleficia guidelines - you register as Infernal to Intellego, so on any mage gathering with overly curious members you can be easily detected and fireballed.
Doesn't that apply only to tainted effects that are active at the moment they're inspected? I remember reading that in the guidelines for Chthonic magic.
If that's the case the simplest thing would be to end any active infernal powers you might have on yourself, as it would seem obvious. Seems to me a fair tradeoff for the "perks" of those powers, lol. But since "accidents" happen and you might want to use some method to mask them, I'd assume that if you can detect infernal powers with InVi you should be able to shroud them with MuVi.
Wouldn't gaining some reputation as a demon hunter somewhat serve as an excuse as well? I remember that the books specify that quaesitors find them rather unnerving and untrustworthy, but I'd believe that they wouldn't just react with extreme prejudice the instant they smell the Infernal on you accidentally.
There is a Creo Vim guideline for blocking magical inspection. The obvious thing to do would be to get a demon familiar and have it cover you. Hermetic Magic can't penetrate demon illusions. Make sure to preemptively cover your foes in infernal tainted magic too, so they register as infernal. Or just a few random magi. Then if anyone does go around scanning random people for infernal taint they'll pick fights with innocent magi. It will discredit it them. Or at least get them to reveal themselves so you can frame them too.
There is no rest for the wicked. Holy God will smite you one way or another. A Flambeau magus will randomly coincidentally discover you and do the right thing by converting you to ash.
A wise GM will recognize that anything flavored infernal will destroy the game and not permit it in the first place.
No sympathy for the devil. Sorry Mick 
Oh, you keep venerating your corpse-god and the instrument of his torture, sir. Reign of the Heavenly Tyrant won't last forever. 
Mick Jagger is not a corpse (yet) nor a god. Kieth Richards may seem like a corpse but he isn't. And is also not God.
I was speaking of the God of Abraham, Issac, and Jacob. Which applies to three different faiths. The "corpse god" comment was sort of mean and hurtful, but I am used to it from gamer munchkins so I will let it slide. But the fact of the matter is that, in this game setting, the Divine is the ultimate absolute power and is utterly good and correct.
Which does lead into Socrates' question about "what is pious and impious". But I will save discussions on real world philosophy and theology for real lfe. For here and now, it is part of the fabric of the game setting. A fictional world that bears some resemblance to reality, yet is not at all reality. A fictional Divine that sorta resembles real world religion, but it is not really the same at all.
Having said that, bend things as you will for your own games and play with ideas of evil if they entertain you. Free country and all. Just saying that I do not allow overtly Holy or Infernal anything into my own games as to avoid conflict and contraversy. I regulate it to background material and try to stay away from providing any definitive answers. Even for pagan gods. I did a short Norse bit, and for my own personal aesthetics I made the Aesir magical beings that care not for mortal attention. But at the same time, there are faeries with these identities that do crave it. Which came first? Neither one did maybe?
Look who's being mean and hurtful now. I asked a simple question of how to hide infernalism without bending the rules. You barged into the thread and instead of saying something on topic decided to talk about how pointless it is to be an infernalist because GM will just randomly fuck you over. That's not helpful.
You are putting words in my mouth that I never said. To say GM would be wise to avoid it is not the same as all GMs will screw you. The bold/underlines you added make no sense and I am not sure what the intent there is. But I will try to keep them in mind.
Also, I wanted to lay out my own personal policy because, just previous to this thread, you requested to join my PbP game. I had never considered that you might consider an infernal path, so I wanted to ward you off from that direction if that is the sort of idea you were planning on using in that game. And I mentioned to do what you want in your game, because I am all about freedom of thought and expression and my feelings do not apply to games other than my own.
I think you are oversensative also. I stated a logical reason why I wanted to put forth my view here. But even if there was none, I hardly consider it "barging in", and even if it was, that is trivial and no where on the same level as making deroggatory comments about the object of my deeply held spiritual beliefs. But even then, I took it lightly and made some Rolling Stones jokes.
That is what we do. We are all gamer munchkins (a term of endearment that I apply to myself also). We tend to shoot off and say all manners of things.
Which brings me back to posting here. Everyone is entitled to my opinion. :mrgreen:
So, have you now lost interest in my game? Or do you think you have the stones to handle the friendly antagonistic banter I am so famous for?
I would disagree with that presumption. There were many clergymembers into herbalism and alchemy, and the situation is made even more complicated because we are dealing with a fiction. I do not believe that "magic" is real or that it ever was. The real "wizards" of history, not proto scientists bu warlocks and shamans an such, were usually liars and hucksters and exploiting people. All evil. If such a thing as magic and spells was real, then society would have operated under fundamentally different precepts. I would add that it is the effects of the Gift that cause such negative presumtions and accusations. A magus with the Gentle Gift would have no such issues. And a wizard who puts his talents to good work can overcome these negative stereotypes.
Albertus Magnus was caategorized as a hedge wizard in 4th edition. And one of the Popes in history, I forget which, was a famous occultist.
So the wuoted statement can at best be considered a tendancy but not an absolute.
As for the OP and initial question, I do not think there is a way to do it. The guy who wrote it designed it with that weakness intentionally, because the Infernal is supposed to be defeated. These are rules for bad guys, not heroes.
It depends 
As stated, the question is a bit too vague to yield a precise, definitive answer.
It might be worth noting a few things though.
First and foremost, a "diabolist" is loosely defined as someone who traffics with demons. A hermetic diabolist can do it all without any special virtues, just Rego Vim. Conversely, a hermetic magus with Chthonic magic and goetic arts is not automatically a diabolist, nor automatically in breach of the code (keeping a low profile might be a good idea though).
Next, there is taint and taint. Roughly speaking, there's a "lesser taint" that registers as Infernal only to those people with Divine and/or Infernal detection powers, and a "greater taint" that registers as Infernal to Magical and Faerie investigation too. If you have Chthonic magic, lesser taint is unavoidable. But as long as you do not "push" Chthonic magic too far (see RoP:I for details), and the only goetic art you use is Summoning, you don't register as Infernal to Hermetic powers and are fine under Hermetic law -- even though people will get suspicious, and will tend to shoot first and ask questions later, IF this is found out. Of course, since only divine and infernal powers can detect this sort of lesser taint, anyone who claims you have it will himself be eyed suspiciously by the quaesitores, and will be hit by a charge of scrying in any case (assuming we are talking of an hermetic magus, acting directly or through an agent -- a hedge magician can just be slain with impunity as long as it's done with discretion). Just try to stay out of the way of holy magi and that sort of people, and push discredit onto them any time you can
If you push your Chthonic magic too far, or you use any goetic art except Summoning... then the outcome is clearly infernal and detectable as such by Hermetic magic. Of course, there have been cases of magi framed by demons before, and one could argue that using infernal power is not technically "consorting with demons", so you are not 100% doomed, but if found out you are definitely in a very bad position. Still, it should not be too hard to limit your use of such powers discrete and out of sight. This is certainly true of Ablating, Binding and Commanding (unless you are caught up in an unexpected battle with demons with witnesses you'd like to keep ignorant). Then, only someone who investigates your magic in your "hiding place" or reads your mind will find out. If you act nicely and avoid rousing suspicions in the first place, the latter is unlikely to happen, because of the privacy clauses of the Hermetic Oath. Of course, a situation where using your clearly infernal powers makes you subject to possible or even certain discovery, but not using them subjects you to death or other grievous loss (including opportunity loss), makes for a great story.
My advice is to really make sure you agree with your troupe or at least your storyguide about how your character's story will play out. It can certainly be fun for the player to play a "doomed" character. But often that's not the player's idea ... and if the storyguide happens to be one who thinks "Well, if you chose any sort of Infernal-related Virtue or Flaw, you freely chose a character who will quickly be destroyed" (and there are surprisingly many bad storyguides like that) your saga will not be fun. This obviously assumes that the character in question is a PC rather than an NPC, which was not obvious from the OP.
In general, summoning demons to cover up your activities by infernal deception is a really, really dangerous course of action, because it provides Hell with a remarkably effective way to blackmail you. It's the hallmark of the really desperate infernalist, or the really arrogant and dumb one.
Marko sounds like someone who has had a rough experience with summoning (which can break games if not handled correctly), though it seems to me that he could stand to be a bit more... tactful in his reply.
That said, you want to know the best way to hide infernal taint from something like Goetia? Look into masking spells from PeVi. You can get them to the point where folks need rituals to detect your magic. If they can't detect it, they can't tell its tainted. Ezz kind of covered the rest there with his rather excellent post.
Ultimately Goetia is a tainted art, if you summon demons with it, you will eventually be caught and marched. Of course, you may not care about that at all. However, if you are simply using Cthonic magic to summon spirits, you can easily make the case that you aren't a diabolist (again, see Ezz's post). As he mentioned, it would be amazingly hard for anyone to prove, which coupled with some masking spells, becomes almost impossible.
So in short, limit yourself to summoning and commanding spells (basically ReVi stuff), mask it and you'd be golden.
And lets face it, in the Order of Hermes, true guilt doesn't matter as much as who you know and who owes you favors at Tribunal.
That's a good point. If your character has a famous and powerful parens and he isn't a dumb poser, he will be able to cast hellfire without problems.
You can also add +5 levels to give your spells a nice appearance. If you're vomiting pretty dragonlike imps instead of little red satanic midget, they will not bring your sodales' attention.
Hi,
First and foremost, I would try to negotiate script immunity. This works better in some sagas than in others, and for some character concepts than for others. In an 'edgy circa 1980s' game, sympathy for the devil is quite popular. In Marko's game not so much.
Second, make sure that my player does not take a Story Flaw that is even remotely related to my fulsome magics, because Story Flaws invite challenges by their very nature. If I were extremely careful, I would avoid taking any story flaws at all, because a Story Flaw as innocuous as a Vow to rescue puppies can become problematic when an oh so cute Hell Puppy turns up whimpering at your doorstep.
Of course, what player, having endowed his magus with Infernal Power, could resist the temptation of nearly free Story Flaw points?
The next line of defense is subtlety. As AM2 core points out, patience is a virtue but subtlety is not, so feel free to go all out. This means making sure no one notices your tainted magic. Remember that Sense Holiness/Unholiness is not automatic, that magi with Intellego need to actually cast, and so on. Remember that killing all witnesses is a completely reasonable approach, as long as traces don't point back to you. Then there are patsies: If you can convince a bunch of villagers to form a gaming group, especially live action VtM, the Quaesitors might not bother to look further than that when they come calling. Better still, be the Quaesitor if you can.
These plans are sure to work and nothing could possibly go wrong.
Anyway,
Ken
I don't do tactful. It runs contrary to my Essential Nature, and I find it disingenuous and patronizing. I do use diplomacy on occasion, but that is a different story.
In my defense, as I said the OP has sent me a PM request to join my game just before posting this. So I wanted to be clear as to how I deal with it. Which is not to deal with it. I disallow overtly Holy and Infernal elements. They cause arguments and contraversy in the troupe. And yes, I have bad experiences with both Holy Magic and Goat Magic. A little bit is okay. Whole hog will not fly.
I do not feel this makes me a bad GM. I am not the greatest, my ego is not that huge. But I do have the largest and longest running PbP game on the Forums. So I must be doing something right.
As it is, I think I ticked the guy off and he lost interest in playing with me. My loss I suppose. I should have been more diplomatic. But life goes on.
Ken, I do like your idea for the hell-hound puppy 