Being sufficiently cool at doing holy things requires a lot of experience points. Like, a really big lot. And holy people don't have a whole lot of ways to get extra experience, nor such awesomeness as Infernal Blessings. Which, I suppose, is part of the point; the power of the Infernal entices both characters and players. I know that I've had at least one character I had no infernal intentions for who I gave in and handed some Infernal Virtues (oxymoron much) to just to finish up the concept. But at the same time, nobody would feel very assured if none of the holy guys were capable of doing holy things to counter the unholiness of the Infernal, so there must be some way to achieve awesomeness. Aside from using things like Baccalaureate to fill in experience from other areas, what are some ways for a holy character to get more experience so they can fuel those Methods and Powers?
Doing so would be a mark of hubris, and for their own glory not God's?
Divine character should be only powerful enough to overcome their foes by the slimmest means. Don't forget, someone with True Faith can use it like Confidence. So, imagine someone with True Faith 3, they can adjust their rolls by +9, so long as it aligns with God's will. He regains his Faith points at the next dawn, too. And then, he can use Confidence on top of this... Adjusting a die roll by +12 or +15 is a pretty big deal...
I have to disagree about the disparity.
The most powerful character I know how to make requires True Faith and can reach Magic Resistance well over 300 and soak well over 60 with ease a single season after beginning. (Those numbers are playing it easy; I can probably pull off three times those.) It's easiest when done with a Hermetic magus (no high Arts needed), but can be done with a magic item. The character also has immense Penetration.
Even avoiding such abuse, there are so many cool and strong things via the Holy. First, remember the lack of negatives (not botch dice, just negatives) when in another Aura, which makes Holy Second Sight the best version of Second Sight, for instance. Want magic resistance and a great Soak as a starting character? Take Guardian Angel. Magic resistance, extra pseudo-Confidence, and access to more stuff? Take True Faith. Even if you're looking at the Powers and Methods, look at Ceremony. It shouldn't be hard to get ceremonies going considering the number of Christians in Mythic Europe. Ceremony is a lot harder for Infernalists.
Yes, the Infernal offers some easy routes, but it really isn't super-powered compared to the Holy.
Ceremony. It all comes down to Ceremony.
Build up a monastery of the Holy Tradition (did someone just say "covenant"?). Get a competent leader, a few high-ranking practitioners, and a clutch of monks-in-training. Use those weird Durations, like Grace or 40. And presto, you have great power! Miraculous effects of level 70 or so are within reach.
Now the thing is the Diabolists also have that Ceremony gig. And their's is 13-witches strong, with Infernal Blessings conferred by the coven to bolster the members to be better at aiding the Ceremony (everyone has an interest in improving the coven!)... so them diabolists still get the better deal. Damn.
... except that Divine Ceremony is a minor virtue (RoP: D, p. 34), while Infernal Ceremony is Major Virtue (RoP: I, p. 84).
Divine win again.
PS: At a glance, the Ceremony rules in RoP: D also appear to be more forgiving than those in RoP: I, in that any number of participants can join.
Still, using Incantation/Diablerie, you can grant your followers Ceremony, if you trust them enough.
No need to let them keep it either, giving them CEremony and Puissant Ceremony with D: Sun can be very useful!
Personally, I feel that the aura interaction modifiers for Divine powers are quite impressive on their own!
Agreed. The Divine cheats that way.
I actually when the Dominion just doesn't quite match up to Infernalist powers.
I do prefer a divine-as-subtle feel than some of the stuff I've heard bandied about with archangels leading armies or whatever.
As to help the holy man out; Divine premonitions. It's all he really needs to defeat any demonic power, if he's smart. Much cooler than salting everyone 
.
Does the Infernal get anything like Ars Notoria (Realm of Power : Divine)? That's gloriously cool in my eyes.....
The holy man probably doesn't get Ars Notoria either, given that Ars Notoria, though Christian, is actually a Magic thing. The cool thing about the Ars Notoria is that it only has one ability, and while it has requisites, there are lots of education-related Virtues to get experience for those languages. Meanwhile, there are no sources of exp that can be spent on actual Divine abilities aside from natural progression exp... At least as far as I know.
Maybe I phrased that wrong by calling it "awesomeness." Sure, some of God's servants are going to choose "burn by holy light, demon!" but Divine characters also have many options that don't serve to boost the self up in any way except spiritually, like gaining insight into Divine will or restoring people from the effects of Infernal onslaught. And even the people who are, say, shooting holy light to burn demons or increasing Soak/granting weapon immunity could very well be doing it for God's glory, for example if they are crusading, or providing an example of true righteousness to those who had become disillusioned with the church's corruption. Sort of a "they're God's main servants, but they're humans who fail; God is greater" message, y'know? Anyway, I suppose I've sidetracked myself.
Even if the case of having foes, at any rate, I don't currently believe that Divine characters have what it takes to overcome supernatural foes, even barely. Unless they're, like, in a church, I guess, but that's sort of like pointing out that most magi will win if attacked within the Aegis of their own covenant; it's not really representative of the character accomplishing anything.
And to be honest... I really just like priests and the like with holy powers. I don't know exactly how much the setting likes them, but it supports their existence, and it's my favorite character type of all, so I'd like to find a way to make them work.
At any rate, fluff quibbles are almost meaningless here. All I'm asking is how to get more exp for them since the Divine is lacking in Virtue exp sources. 
Ars Notoria is in fact Divine. It just happens to be a form of Divine magic that requires the Gift for some reason (probably because it's old rules). You're right, though, that holy characters aside from Holy Magi won't be picking it up because it can't be Initiated.
If you're asking for progression in play, the first thing I can think of is that holy characters ought to adventure every chance they get. There are ways to be a holy lab rat (more on that later) but holy PCs operating on the Companion or Magus level should not resist going out and getting stuff done; unless a story is Hermetic-specific (such as a Tribunal meeting) or necessarily involves doing something blatantly sinful, a holy PC should be able to get in on it and earn Adventure XP from it. Consider taking Independent Study as a Virtue. Also, Divine Gloom is a good way to reshuffle points from easily-relearned Academic Abilities into your divine knowledge.
Now that that's said, The Church has a lot of ways to address this very discrepancy; it's Catholic-focused, of course, but adapt as needed for other faiths. For anyone, Worship (p.27) has a Source Quality equal to the aura in the church. If you have a good Divine Aura, this can be pretty good practice (not as good as studying from books or vis, but that's the power of the Order of Hermes for you). Sufi Muslims should definitely take advantage of this if they're not being taught, as their tariqa is probably based around a holy site (and as of 1220, with the Hagia Sophia corrupted and Jerusalem in Muslim hands, Muslims have the best selection of holy sites to work with). A Mazdean PC should also be able to get a pretty good fire going: if you can light an Atesh Behram (TC&TC p.86, this requires a story, and then you need to make sure your ervads keep it pure, but it can be done) you can reasonably expect a +10 Aura.
If you really want to be a powerful saint, Monistic Mysticism (p.23) is the gold standard in awesome. It allows you (with a high Concentration) to Practice at levels equal to or exceeding Worship, and it lets you take more Virtues from Divine Ascent; of course, being Twilight Prone is something of a downside (Monistic Holy Magi Ex Miscellanea might see it differently...). Contemplative Mysticism (p.21) is somewhat weaker; if you are really dedicated to piety, it can still get you a lot of Virtues, but it doesn't give you a route to improving them through XP. Both kinds of mysticism generally imply that you're going to be developing your abilities on your own, however, which means that you won't have any Ceremony-servants unless you teach them yourself (which will be difficult unless you are up for creating a Mystery Cult from scratch).
Finally, if you're a Mazdean, Cathar, Sufi, or Kabbalist, you may have textbooks, teachers or both. Use 'em.
If you want to specifically be a priest with holy powers, take a look at Zoroastrianism in The Cradle and the Crescent; the mobeds are fairly close to D&D clerics in style if not in specific powers. You're swimming upstream because Zoroastrians need to deal with a lot of crap for being Zoroastrian - but as any magus ex Tytalus will tell you, that's a recipe for fast growth through Adventure XP.
... Whaaat. Ars Notoria is Divine!? I... Wow. I need to be more observant. And this game needs a better explanation for requiring the Gift for a Divine power... 
In more ways than one, for that matter. And here I thought I'd get most of what I needed from RoP:D. So I'll be saving some money and picking up The Church, then...
Oh, the Cradle & the Crescent! I just picked that up recently after reading another thread last year about how it's good for summoners. Dual benefits! Time to crack that baby open.
Thanks again.
Pg. 100, RoP:D: "The actual powers of Ars Notoria are considered Divine for determining how they interact with others."
However, Ars Notoria is a really, really weird power in general, so understandable that it got confusing.
I think the OP really conflates together two questions, which should be best kept separate.
The first is: are Infernalists too strong? Not just in relation to Holy men!
The second is: are Holy men too weak? Not just in relation to Infernalists!
Let's start with Infernalists. Infernalists are phenomenally strong. This makes sense, for Hell must really offer a suitable price for your soul! It's also good if you have Infernalists as antagonists, because you can make them a strong challenge even to archmagi following the plain RAW. And if you play Infernalists? Then, you do risk having them feel overpowered; but it's true only in a mixed group, and even then only if the Infernalists are not a small minority having to hide their nature from anyone else (which can be very effective at curbing their power).
Let's move to Holy men. Let me repeat, it's not Holy men vs. Infernalists, it's "are Holy men too weak to be producing interesting effects in play"? Personally, I don't think that's the case at all.
First and foremost, as people pointed out, there's Ceremony. Ceremony can really give a huge boost. Even if the Holy man is the only person with Ceremony, leading half a dozen people with +2 in the appropriate characteristics and no Methods or powers can give a substantial bonus. But if you push it, and write up half a dozen followers of the Holy man each with Ceremony at 5... I once played an Outlaw leader (pious, rebelling against an evil overlord) with half-a-dozen followers each with Ceremony, and I can assure you that it did feel a bit overpowered - the main "opposition" was due to moral quandaries and the strictures of righteous behaviour than to "it's just six of us vs. sixty opponents".
Then, there's True Faith ... including the True Faith you get from Relics, which is way cheaper! That can give a huge boost, not only to Methods and Powers, but to any ability you use (including e.g. Charm or Area Lore or Enchanting Music or Medicine). A Major Relic (with 3 faith points, and a Major Virtue) can give you +9 to any one roll, once/day (as long as the purpose is pious, obviously).
Then, there are Saints! Saints are fairly easy to invoke in the right circumstances; and they come with a massive package of miracles. Saints are what makes Christianity "mechanically" superior to Islam in Ars Magica terms. Muslims can invoke their holy forefathers, but only at specific places, which is a huge limitation (on the other hand, the Qu'ran can be invoked everywhere, and it does have a fairly extensive suit of powers). I'm not really dealing with direct miracles, because those are very much subject to GM fiat, but they could easily enter the picture too.
Finally, holy men are often sanctioned by society. Ok, there are heretics, but most of the time mundanes will support you if you are a holy man, instead of trying to burn you at a stake as they'd do with an infernalist. This may seem relatively minor, but it does have a noticeable impact from my experience.
From my experience, the three main limitations that Holy characters face are:
a) Strictures of behaviour. Basically, there's lots of stuff you can't do, and in fact you wouldn't do even if you could, because you are pious. It's both an interesting challenge, at times, and something that makes the characters a bit boring at others.
b) The total powerlessness if and when God tests you and temporarily strips all your powers away. Oh my. That's one of the things I really, really hate. I mean, it's absolutely appropriate thematically, but as a player, I find it terribly annoying.
c) The fact that you can't toss around miracles like a Flambeau tosses pila of fire. Faith points "regenerate", but slowly. Ceremony takes time. Meditation and Purity take time. Saints and Miracles are less likely to succor if you've called upon them very recently.