I'm tempted to put the resurrections into the realm of "magic," BUT, I think that there can be something divine there as well. The answer is: I honestly don't know.
On the magic front . . .
Blood, life, and death seem to be keys that can help unlock the secrets of magic. Blood for Drogo’s funeral pyre when Dany hatched her dragons. Death of the witch that made his wound fester. The life of her unborn child. One could argue that these three things are what brought the dragons to life. No god necessary, no divine intervention if you will.
As for Donarrion, perhaps these elements were all present when Thoros revived him (not once, but multiple times). It’s hard to say. Perhaps that was more divine-related. But, keep in mind that not all who are raised by these “priests” come back the same. Dondarrion is unique so far. Others have not done so well (and there will be others). Furthermore, how does this relate to the White Walkers beyond the wall? Is it all magic? Is only some of it magic? Is it a necromantic thing? Or something else?
I think an argument can be made for either religion or magic to be perfectly honest.
For me, I am intrigued by both ideas. On one hand, magic would explain a lot and it shows us that there is a price for magic such as this. On the other hand, the notion of the gods being real (or facets of one true god) are interesting as well, considering that the gods come across as very ominous, strange, mysterious, and very very creepy in the books—especially when you get to the Faceless men (Jaqen H’gar from season 2 – the assassin/shapeshifter who helped Arya).
Even with The Lord of Light’s seemingly good side, there is an equally disturbing bad side. Need of royal blood, shadow-assassins, etc.
I think you can see why both would fit.
Excellent conversation, though, and something I’ve always had ideas about myself.
As for who R'hllor is, check out this wiki site. It does a great job at explaining.
http://awoiaf.westeros.org/index.php/R'hllor
Some spoilers, so beware. A snippit:
The Religion, is based on a dualistic, manichean view of the world, R'hllor, the god of light, heat, and life; and its antithesis the God whose name should not be spoken, the god of ice and death. They are locked in an eternal struggle over the fate of the world, a struggle that according the ancient prophecy from the books of Asshai, will only end when Azor Ahai, the messianic figure will return wielding a flaming sword called Lightbringer, the Red Sword of Heroes, and raise dragons of stone. According to Martin, this faith is roughly based upon the fire worship of Zoroastrianism. This religion's dualistic aspects of a good and an evil god are also inspired by Zoroastrianism, along with the Cathars of Medieval Europe who were annihilated during the Albigensian Crusade.
There is more in the link.