Is there a better way to handle the dominion than by a simple mechanistic drain on spell casting totals? Thematically/stylistically I am not convinced that this is really the best way.
I was thinking, surely the strength of the dominion be proportinal to the chance and strength of a reaction to impious people and events? Examples include Magi casting spells for selfish or indeed anti-Christian reasons. Perhaps some sort of stress die + Magnitude + Dominion, perhaps with an ease factor dependent on how blasphemous the action is. The purer the act the higher the ease factor. 'Success' indicates that the Divine has nullified the spell (or in extreme cases perverted it).
Part of the idea is that people have Free Will , even magi.
Taking an active stance on moral actions would be interfering with that.
If the Dominion did as you suggest , what about Infernalists or anyone with an Infernal Might Score?
Those magi with Chthonic Magic would also incur whatever penalties apply to The Infernal , regardless of Piety.
There are no crap ideas that cannot be used as manure for growing new concepts.
(he says redactively)
We sometimes make auras affect spell casting in weird ways. A dominion aura might not hinder you if you are battling a demon or trying to heal someone altruistically (not because he is your shield grog and you need him to act as flak armor for you). It might even provide a bonus in those cases. The same for infernal auras, but in that case it is a bonus to bad stuff. It goes down yto 100% SG fiat (the paths of the lord are inescrutable) if this happens or not, and we do not use it miuch, but have found that changing it from time to time makes magi more aware of the power of the divine instread of God being "that jerk upstairs that just annoys us because he fancies it".
I reimagined Dominion for my upcoming saga, so that instead of penalizing spellcasting rolls the Dominion provides magic resistance equal to 3x the divine aura. The idea being that God/faith protects people. In some ways, this makes the Dominion more powerful vs magic (and it messes with 5th ed's "parma is king" philosophy but I long since rejected that in my sagas) but at the same time it adds an interesting dynamic... since anyone can choose to lower their magic resistance (and by extension the resistance for their property and land), there is a potential social element in convincing people to allow you cast spells on them. It's working pretty well so far.
I like the idea of having the interference of a divine aura a bit more stylized, but I would probably keep the system as is and occasionally introduce some weird effects via caveat like Xavi said. One reason is that I wouldn't want to change to how Magi have to deal with low level auras in towns and cities. Where magi have to stick to smaller spells they can cast very carefully. For more powerful auras I certainly like the idea of high magnitude effects calling down the wrath (or at least attention) of God no matter what the Mages skill level.
I think the Divine would defiantly react differently depending on the intent of the caster. But I'm not certain I would tie it to one religion's morality like Christianity For me I like to highlight the fact that the dominion isn't tied to any specific religion. It associates it's self with any sincere worship of a single benevolent creator.
if the people's opinion is that magic is bad, the divine will give malus, as it is in most part of Europe
In celtic places were druidism has become christian, magic is accepted as possibly being good and without malus for casting spells in those celtic tainted divine auras.
For what its worth, the game has a couple of decent reasons for the Dominion penalty.
The most important is philosophical and theological: "God is a jealous God." The Almighty's Dominion aura is there as a dampener upon the pride of magi and the other auras. It keeps them safe, but also is a constant reminder of the inferiority of all other power next to the might of God.
The jealousy of God is not simply moral in the sense that we understand it in the modern era. The God of medieval Christianity is not the God of modern Christianity. He really does believe in impressing upon people the scope of his power, and has absolutely no tolerance of direct competition. So "being nice" or "helping people" is not sufficient to be free of God's jealousy. One must follow the law rigorously and devote oneself to God's will, as writ in Scripture and the Doctrines of the Church.
That's why the game has Holy Magic, to show just how far a magus would have to go to accord with God's will and thus merge one's magic with the Dominion, and overcome the restrictions thereof.
That's why I'd say that if you want to straight-up overcome the Dominion by the purity of actions, Holy Magic should be necessary.
If you wish to rework the Dominion in some way, such as by giving Magic Resistance in Dominion auras, that would work. Otherwise, however, you'd have to throw the entire Medieval Christian Theology out the window, and that might rob the game of some of its flavour.
You know what I think you are completely right. One of the things that attracts to Ars Magica is the idea that Christianity is 'right', or at least appears to be. It is a refreshing change from the constant moral ambivalence of the modern mind. As an atheist moral reletavist/nihilist I love this! For me to allow it to be tweaked or negotiated around is wrong. It should be absolute and non-compromising. Magi should not be able to ameliorate or negotiate with the Divine, but suffer it or submit to it.