Note there's one option not explored; if Ars Notoria is part of a Holy Tradition's favored abilities, any character could learn it as other supernatural abilities (including arts) don't restrict learning of favored abilities. Though you need True Faith and can only join one Holy Tradition, a tradition with Holy Magic and Ars Notoria doesn't seem beyond reasonable.
Are the arts really part of the supernatural abilities calculation? The text in the corebook seems to imply they are separate, but I guess that would make it near impossible for a learned magus to gain anything at all from holy societaes.
If so, that opens some interesting doors. The talk about designing a specially built character to make the best of this is interesting, but not super applicable to my case as I'm not gonna rebuild my magus in the middle of a campaign. I do have some control over the Tradition I'm going to join though, so If I want to add another wrinkle to this, that's an interesting path. Especially since, having read through Ars Notoria, it seems Really strong. The first two rings alone giving 200xp and +3 to two different stats seems absurd. Needing to take some time to learn the languages is a pain, but made up for and more with the bonus xp.
Also, if I'm reading it right, you could keep the bonuses going indefinitely without any loss as long as your ars notoria score is reasonable as
You may miss up to ten days from any laboratory activity, and make up the time by working harder during the remainder of the season. There is no penalty for this.
And
Future recitations of a known ring do not take as long; it takes 10 days minus your Ars Notoria score to perform an oration you have previously recited.
So as long as you can make the recitation reliably, you'd only be limited by the number of rings you were trying to keep active.
@GayWhiteBoy Characters opened to the Hermetic Arts must subtract 15 or the sum of all their Art scores from the Source Quality, whichever is greater (that is, whichever results in a lower source quality).
Initiating the Ars Notoria might be possible so long as you still initially learn it from the text (which probably precludes the +3 bonus to the script from having a teacher, but the language and artes liberales requirements might help with that).
Depends on the way the rules work. According to John and somewhat corroborated by this thread (notably the bit where each art is can be thought of as as a supernatural ability that is favored by hermetic teaching), if Ars Notoria were a favored ability of the holy society my magus intended to join, he wouldn't suffer the penalty incurred by the arts.
And, frankly, if it doesn't work that way, converting a magus to holy magic partway through would be basically impossible. Learning any methods or powers or even holy magic itself would require an impossibly high source quality for any halfway capable magus unless favored abilities waive the penalty for arts.
you forget one thing: Initiation.
If your magus joins a tradition, they may have initiations scripts which will grant him the requisite virtues, with associated abilities 
If your magus has True Faith, he'll benefit from the Favored Abilities of the tradition, yes. But most magi undergoing that sort of conversion would instead initiate the abilities and not have to worry about the penalty for that reason, unless they gained True Faith in play (to be fair, this might be a fairly common reason one seeks out a Holy Societas to join in the first place.)
Yes. In general, Holy Magic is not an easy way to power. Which is good, in my view: only those very few magi with other motivations will pursue it.
Make sure you milk being good for all it's worth. Pious Major Personality Flaw? That's 3 Virtue points handed to you for roleplaying the character in the way you'd play him anyways.
Keep in mind that raising mundane skills is generally really easy for a magus in a mildly organized covenant. That's because you just have to hire/train a competent mundane teacher. In general, that's a way easier path to high theology than Ars Notoria.
Also, though this may be open to debate, I'd rule that using Performance Magic - particularly a Performance Magic that is appropriate, such as singing/playing religious hymns - would avoid the -15 penalty, as you'd not be using "Hermetic words or gestures".
Yes.
It certainly might, depending on the activity that accrues a light wound or long-term fatigue level.
Casting-through-fasting means no fast-casting! Self-inflicting a light wound through flagellation would probably allow you to cast a spell normally (though again, probably not quickly enough to fast-cast it). Self-inflicting a light wound through some enchanted device, or a spell kept hanging through ReVi magics, should probably cause no appreciable delay.
It's a bit tricky to adjudicate, but you may consider fatigue loss from Ritual spell-casting to qualify.
Also, I'd allow a harsher version of Painful Magic (that either causes long-term pain, or actual light wounds) to qualify.
You have to choose one or the other ahead of time. More precisely, you have to choose ahead of time how you'll be casting. You can use religious ritual (which may fail to "mystically substitute" for Hermetic words and gestures if you do not roll high enough) or you can use an action connected to one of your Methods.
Time is everything. This can be a huge loss for the experienced mage, particularly one who has invested significant effort in mastering his spells. Unless your mage focuses on spontaneous magic or enchanted devices, it's one of the things that makes you want to gain Holy Magic early.
And keep in mind that you'll also have to "relearn* Magic Theory as Holy Magic, in that using Magic Theory (rather than Holy magic) in the lab is as sinful as casting "old" Hermetic spells (RoP:D p.86).
Maybe. Note that it does not affect learning from vis, from djinni, etc.
Yes, if you are such a good writer that your tractatus are really worth something for your covenant. This depends a lot on the setting assumptions, but in a sufficiently "open" Order where books are frequently traded, the vast majority of magi are not really "good enough" to write books others will take the time to read.
Eh... this can be a hindrance. It's up to your troupe to choose how much to bring it to the fore. Not only you'll be the one that demons and their servants target, but the stereotypical friendly faerie will often not be that friendly to you.
Well, instead of looking at time and resouces lost, try to compare for the same time and resources, the oomph that the Holy magus will have. Generally, it will be significantly less.
It's extremely good if you are frequently in Divine Auras, not just because you are now getting +Aura instead of -3xAura, but because you are avoiding extra botch dice too!
Yes, this is one of the big big perks of Holy Magic. It makes Holy Magic perfect for the Hermetic Ritualist - say, a Healer, or someone who conjures big, powerful, flashy effects.
page 67, actually, and yes, it's an error: it definitely should be a Ritual. That said, given the existence of theriacs, the ability to restore fatigue is less impressive than it's often portrayed.
It's not just an issue of Aura bonus/penalty. For example, a magus can't usually extract vis from a Divine Aura, but a Holy magus can.
Well. There's a big catch here, that is often underestimated. You need to have those Methods and Powers in the first place, and then you can produce just the same non-Hermetic effects that a mundane character - say, a companion - with those same Methods and Powers could.
So, I do not see Holy Magic as allowing a Hermetic magus do have access to a broader range of powers. Instead, I see it as something that allows a Holy mystic relying Methods and Powers who also happens to be a Hermetic magus to increase the oomph of his Methods and Powers by leveraging his knowledge of the Hermetic Arts: even if your Purity and Intervention scores are 1 each, you can easily raise the dead if you are a good enough master of Creo, Corpus, and Mentem and you research the appropriate Ritual (i.e. you are sacrificing the built-in "spontaneity" of Methods and Powers).
You are missing several things. Invocation allows you to use vis to boost any Method+Power total, even those not relying on Invocation. Purity allows you to apply your Confidence and Faith points over and over. Some traditions have Virtues that provide additional bonuses (e.g. Kabbalists have two Minor Virtues that allow one to add, respectively, one's Artes Liberales and one's Philosophiae to one's Kabbalah totals).
But the big, big, big one is Ceremony. Divine Ceremony is a minor Virtue, but it can generate large casting bonuses if you have it, and you have a few followers with good enough characteristics. It can generate huge bonuses if you have a small cadre of followers with some experience in it. I once had in my saga a companion who was an Outlaw leader - a pious knight who, with half-a-dozen faithful followers (pious knights themselves) had decided to rebel against his evil lord. We had always allowed characters with similar "leadership" V&Fs to design their followers as grogs, and made no exception in this case. Well, the Outlaw leader and each of the six followers had Ceremony with a score of 5+1, and appropriate Characteristic bonuses of +1. That meant a +42 bonus on the leader's Power+Method rolls... but also on the Enchanting Music roll one of those grogs made to sway a moderately powerful (Might 40) dragon to aid them in their quest.
None is truly game-breaking, but all are quite useful, in particular because they can be used with Spontaneous Magic. A number of them allow you to boost your Duration slightly, "for free" (Sabbath, 40, Office). Devotion is a very useful reskinning of Performance from Performance Magic (a Minor Virtue from TMRE): with it and a small cadre of mundanes you can effectively sustain an effect forever. Grace can be similarly quite useful with the appropriately pious targets. Sin and Faith are incredibly useful for those big, region-encompassing Rituals Holy Magic is so good at.
While Holy Magic is more about exploring the "pious" magus than a road to earthly power, for those who like the mechanical challenge:
- Minimize the disadvantages. Get an exceptional mundane trainer in Theology. Have good Presence (which helps in other ways too, e.g. with the new Range). Join some group of Holy magic that can provide you with lab texts and other books or, failing that, be good at independent study. Learn Holy Magic early.
- Maximize your advantages. Specialize in Big, short Duration Rituals. Milk the extra oomph from new RDTs.
Yes, as mentioned above, you can use Holy Magic instead of Magic Theory in lab activities. Not doing so is equivalent to using "old" versions of Hermetic spells instead of the new ones: it's sinful, and may eventually cause your Holy Magic to stop working.
No, if you are a Hermetic Magus and have strong Arts to fall back to.
However, keep in mind that for any non-Hermetic effect leveraging a Power, you'll need to learn a Ritual (and spend the appropriate vis/fatigue). You do not incur this limitation if using the Method+Power combination directly.
In principle yes, but remember that stuff that breaks hermetic limits must be a Ritual, so you can't instill it into an enchanted device.
This is all awesome, thank you!