For quite some time our sagas have not seen a "holy" PC: a companion or magus heavily invested in the Divine. We have discussed this at the gaming table, and we came to a conclusion that it has to do with the mechanics of the game. Basically, it's two things.
The first problem is that holy PCs tend to be either overwhelmingly powerful or underwhelmingly inept. If you are a novice player, or one with little taste for mechanical optimization, it's really easy to create a "holy" PC that can't accomplish much at all -- most "example" PCs in the books would qualify, for instance -- which can make playing one a bit frustrating (you'd want to have your major Virtues to have a major impact). But in the hand of an experienced player, a "holy" PC can easily overshadow most other characters in terms of mystical weight, including magi (which is particularly bad in Ars Magica). The main culprit is Ceremony, but also True Faith combined with Major Relics; and also to some extent Miracles, invoking Saints, gaining Virtues through Mysticism etc.
The second problem is more subtle, but no less important: somehow, all holy PCs tend to be very one-dimensional. In real life, faith is a struggle. But in our games, holy PCs just stay holy, no matter what sort of temptations get thrown at them. The reason is that, when holiness is the core concept of a PC, the player will not give it up ... even if the character gets tempted, because for the player giving up the character's holiness is like giving up the character. A SG can threaten these PCs with death, and they'll call the bluff and stay true to their faith (which in ArM5 provides a pretty good shield against death, in the right hands) -- at which point the SG can either kill them off e.g. with a Crisis of Faith (which feels somewhat unfair) or let them be saved (which makes such characters even less likely to budge in the future).
The result is that the few "holy" PCs we occasionally see are grogs, for which being one-dimensional is ok, since they rarely take the centre of the stage. This feels a terrible shame, because the struggle with faith can be a lot of fun to play if supported by the right mechanics -- and ArM5 is generally really good with mechanics. Is this just us, or has anyone else faced similar issues?
Just being "holy" - and hence powerful or inept - is indeed a pretty boring concept for a character. He might far rather meditate and pray, just like many magi would never leave their labs. Throw him into the world nevertheless, where moral challenges abound: these are generally more consequential for him than for the magi!
Perhaps you should design adventures, where the "holy" character has to work alongside the others to achieve a common goal, which is not plain monster-slaying?
I recall fondly work on disaster relief, which a covenant undertook alongside a Franciscan convent. Even while there were no characters with True Faith involved, still two quite different approaches to world, life, fellow humans and methods of problem solving had to be coordinated - which made the Franciscan character quite real, and very useful too. Nobody tagged him as one-dimensional any more after that.
But reconciling a city in violent discord, straightening out a corrupted monastery, founding a hospital for pilgrims and strangers, or mediating in a peasant revolt might be as challenging, depending on the characters involved. Just don't have the problems be resolved simply by finding and dispatching the demon culprit!
Indeed, Holy characters can get really OP if min-maxed, and that's a design problem. In particular, you probably don't want to allow a PC to have Ceremony slaves, both because Communion-slaves aren't grogs, they're apprentices, and they cost you a season a year, and more importantly because Holy Magic bypasses the lab work that puts a damper on having a wide variety of giant Mercurian rituals. (Imagine a Diedne version of Communion that could help Spontaneous Magic and allow L50+ spontaneous...)
As for your second problem, I think that the idea of constantly testing faith, while very medieval, doesn't work well for a game. (Also, if you have Divine Form, you're immediately whacked for sinning in a Pavlovian fashion. Bad design again.) Instead, I would look at a holy character more like a well-played paladin in Generic Fantasy RPG. Their holiness limits the options available to them - they can't do something that would be a mortal sin and have to try to stay away from venial sins, They're often prevented from "just murder the problem," they have to bear up with equanimity when others treat them like dirt (where other magi might rule by fear) and they might wanna stay away from Tribunal. These limitations help define the character and give a starting point for play, but leave room for development in how the player works with them.
Then when the PC ends up in a situation where there doesn't seem to be a way out without compromise? Bam - you have a test of faith that arises organically from play.
I haven't got as much experience at this as you, Fafnir. So my perspective may not be worth much.
I've had good experience with magi who are pious Christians, but not fueled by the Divine. They don't have True Faith. Their struggle to rationalize and practice their faith while also being Hermetic magi has been interesting and fun. As the GM, this has been visible to me in two primary ways: the intrusion of pagan rituals on the life of Christian magi, and the reluctance magi to accept Christian society's strict gender roles.
In the first case, we're talking about magi and covenfolk performing pagan ceremonies, perhaps honoring Irish "gods" or faeries, but also things like the Oath itself, which is the Oath of Hermes, and therefore intrinsically pagan. Most Christian magi just try not to think about these things.
The second issue manifests when female magi refuse to be subordinate to men, and when gender gets confusing through something like the Transvestite flaw. There's a lot of strong opinions here about the non-historicity of the Order of Hermes, how it's really a very modern institution in a medieval world. But I personally find that it's distinguishing characteristic. This is a feature to me, not a bug, and I have always thought of the Order in this way, as a modern anachronism, and that's what makes it distinctive. So when a Flambeau has a squire who turns out to be a 15-year old girl, which the Flambeau knew all along and kept quiet, well, the local priest patiently explains to everyone why this is against nature. And he's got a lot of justification for his views. But the magi, even (or especially) the Christian ones, just want to Pilum of Fire him.
This is where magi intersect with the Divine in my stories, and where they experience crises of faith.