Just wanted to check, are the metaphysics of belief still considered part of ArM? Fifth edition seems to intentionally avoid the question. Or is there more objectivity to the metaphysics of Mythic Europe in its latest incarnation?
Basically, previous editions of ArM suggested that the type of aura in any given area depends on what the locals believe. So, in Universities, the aura of reason developed because individuals there believed in a reasoned, logical and entirely predictable model of nature. Divine auras are wide spread purely because people believe in the divine, and not because of any intrinsic divine authority, and so on. I'm not sure it was ever fully official, but I believe this belief-centred model was suggested in Wizards' Grimoire.
Is this approach correct for ArM5? The reason I ask is because the way the Divine has been framed, as the ultimate universal authority, suggests that divine power is subjectively REAL, and that divine authority does NOT depend purely on believers. Instead, there are believers because the divine authority is there and responds to them!
The auras rise because of the activities in the area and the presence of focus in the area. If a lot of magic is performed somewhere, it is likely to permeate the area and a magical aura might spring. Same for the other auras.
In the case of the divine, IIRC it is defended as the divine protecting its flock, and radiating out of the churches. It is the people belief in god that creates the aura through the performance of the church rites in the area, but it is usually encouraged by the presence of relics, shrines and the like.
About the reason aura, remember that you can go out and experiment direct magic and see demons and ghosts in mythic europe,, so it is a senseless aura: reason tells you that the supernatural exists out there. Just a footnote
As far as I can understand from the books, it seems that actions of the people (and other beings) in the area strongly contributes to the aura.
On the other hand, divine actions has little to do with the believers - that has to do with HIS will...
Spontaneous Auras still exist, however the Divine and Infernal can both create their Auras during worship ceremonies on purpose through devoted agents (clergy, for example). Rules for the rituals are in The Divine and the Infernal.
I've always found the Ars world view facinating, and have played in campaigns where local belief and great or no effect. As far as I can tell the RAW establish a universal law system that more or less reflects many beliefs of the day, however there seems to be little support to local areas being able to impose their different world view on the overall world.
That is, if a local area doesn't believe in magic, there's no evidence that it wouldn't work (even in the way True Reason used to work). Or that if one culture believes you can reach the moon by sailing West that you could by starting in their waters.
There has been some indications that local beliefs can ADD TO the basic laws, such that if local custom says that certain potatos contain fairy babies, they do indeed in that area only.
The mechanics behind this, I have no idea, except to say the feeling seems to be "whatever works to make a good story."
At a grand level the answer is no. Reality causes beleif not the other way around. The beleifs and actions of God explicitly created the world and the beliefs of people are not powerful in a mage the ascension sort of way.
However as others have noted above the actions of people can create and alter auras. Areas of pagan worship bcame faerie (proabably) or magic (perhaps occationally) as appropriate because of the actions of people. Churches and cities have the protection of the dominion in large part because of the actions of people. The auras of covenants tend to increase over long periods of time because of the magic practiced there.
Futhermore as others have said the realm of Faerie tends to be shaped by the beliefes of people. This is the result of faerie nature but it is a sort of belief defines reality type of thing.
The fifth edition core book provides two theories held by members of the order concerning the big pictue metaphysics as to how the four realms relate to one another. One was generally based on Plato and one was generally based on Aristotle (but both were I understand created by the authors who were trying to find a way to harmonize the setting of ars with period philosophie).
I otherwise agree with Erik, and only wanted to add a single comment.
Reality causes belief - that is the game mechanic (and the one that distances it from Mage or Ars 3rd ed. for that matter). So in the world of ME reality defines belief, but as meta we often define the reality of ME on the beliefs of medieval people - or at least som of them and often very biased, but that is what gives ME its mythic yet tangible feel.
The two sound alike and can be confused, but I think it is important to be aware of that distinction.