A other thing we need to talk about is how far we go with the incomprehensible between hermetic magic and holy magic as it affect how we will spend our BP for the covenant.
I already started the question here: https://forum.atlas-games.com/t/rop-d-questions/11640/1 because what is writen in the book is unclear and there is no official errata so far.
1.) One can read the text writen at RoP:D 67 that only the lab text are of (nearly?) no use between Holy and Hermetic magic. (Should be quite easy to handle for our covenant)
2.) I personal think the text was aimed for Art and Lab text that are incomprehensible between Holy and Hermetic magic. (Need some more planing with our covenant BP and books but should still ok to handle)
3.) We also could say certain ability beside the art and lab text are incomprehensible but for this we need to be clear right from the start about each single ability (Would need quite a lot of preplaning for this)
4.) The most radical interpretation would be a complete incomprehensible what means all Ability as well. (That would be just crazy because then even a Theology or Artes Liberales summa could be incomprehensible depending who have written it)
Suggestion: The subjects that are mutually Incomprehensible between Holy and secular mages are: all 15 Arts, and the subjects of Finesse, Magic Theory, Parma Magica, Penetration, and any Spell Mastery ability. Purely academic work, such as Artes Liberales, should not be affected, as you mention.
Holy Magic use Holy Magic instead of Magic Theory so there is no learning from the other ability.
Pentration and Finesse bring up the point that they are the same for all hedge magic user and ancient magic as well so why would it incomprehensible for Holy Magic? Beside that if someone without gift write a Summa how would you want to handle this book then?
Because Mastery belongs to a specific spell and Holy Magic user have to complete reinvent their spells I would even say its complet incompatible like the lab texts (Half the lab total means its as easy to invent the spell new as using a lab text)
Leaves Parma Magica what is a clearly conflicting chase as it is said to be a ritual but in opposite to your spells and rituals you keep your score in Parma Magica when you learn Holy Magic. But given that for the Arts I also didn't need to spend seasons to relearn them it could as well incomprehensible if we go for arts are incomprehensible.
Here are my thoughts on Holy Magic and incomprehensibility.
Some things are easy to figure out. Arts clearly fall under the incomprehensibility rule, while Academic, Martial, and General Abilities do not. OTher things are not so easy. Arcane Abilities are where the real confusion comes up. In my opinion some of the Arcane Abilities are incomprehensible, and others are not. For ease of analysis I'll break them up below.
Magic Theory: This is the easy one; it should be incomprehensible
Code of Hermes: Strictly a knowledge-based ability; should not be incomprehensible
Lore Abilities: Strictly a knowledge-based ability; should not be incomprehensible
Finesse: This is a skill about using magic in precise ways. We can teach it to our grogs for using magic items. I'm persuaded that it should not be incomprehensible to a Holy Magus
Penetration: This is a skill about using magic more effectively, i.e., being able to give it more punch. It seems to me that this skill should be independent of type of magic. I'd say that, barring there being a holy option for Penetration that Penetration should not be incomprehensible.
Parma Magica: Here's where things get murky. PM is based on conventional magic theory, not holy magic theory. But there is no Holy Parma Magica that a Holy Magus can learn. I'd say that, barring there being a holy option for PM that PM should not be incomprehensible.
Mastery: I agree that since Art and MT are incomprehensible, that Mastery for conventional spells should be incomprehensible. Mastery for Holy spells should obviously be comprehensible.
lets wait what silveroak say on this but right now it looks like we go Arts, Lab texts and Mastery Incomprehensible, the rest not.
About Magic Theory I only can repeat Holy Magic use Holy Magic instead of Magic Theory and complete abound Magic Theory. So yes, the old Magic Theory is still there but wouldn't be used by a Holy Magus.
So over all Magic Theory can't be incomprehensible as it wasn't changed instead something different was learned with the ability Holy Magic. But if you really want the ability Holy Magic just incomprehensible to the ability Magic Theory that is also fine with me.
Keep in mind that holy magi can still learn from hermetic soures but at half rate, and what they write is simply considered incomprehensible (as per the flaw) to non-holy magi, so a complete separation is not required.
I believe that finesse is clearly not affected, any more than magic lore would be- these may be arcane abilities but they are usable by anyone, the same way penetration can be studied across traditions. Parma Magica on the other hand is exclusively hermetic, and would, I think, be subject to the same limitations as the arts- retained at full value but learned at half value from hermetic sources. What is not cleat is whether that halving of values includes the level or just SQ- my inclination would be to go with SQ only.
So Incomprehensibility is limited to Arts, Parma Magica, and Spell Mastery abilities. That seems reasonable to me. If Adauli's character reads a Spell Mastery Summa on Pilum of Fire (Level 3, Quality 10), he can accrue 5 points per season studying until he reaches level 3 (30 XP). Only Source Quality is halved, not Level, for Summae - a Holy magus reading a secular book has to read more slowly, constantly translating terms to work with his own understanding of magic and the Divine. Likewise, a secular magus has to continually remove references to the Divine to work within orthodox Magic Theory.