Does Melisandre really worship R'hllor, or does she merely use that belief?

Question

As far as I know, there once was magic in Westeros, just as there once were dragons. For example the Wall is said to be built with the help of magic. I don't recall anybody sneering about it, but I get the picture that people "know" and "acknowledge" there once was magic.

When Stannis releases the Onion Knight they discuss the things they witnessed and Stannis concludes that the Lord of Light must be real. At least in the TV show, the discussion is concluded with a (probably rhetorical) question how all this can be if the Lord of Light wasn't real. I find it rather strange that people never even suggest the return of magic. (Especially after they seem to have established a very peaceful co-existence of monotheistic and polytheistic beliefs, but I'm straying ...) With everything I think I know, I'd rather say with the "church" of the Lord of Light magic finds its way back into the world.

When Thoros of Myr told Melisandre, that the Lord of Light had brought back Beric Dondarrion from the dead 6 times, she was startled and said: "you shouldn't have that power". The interesting thing is, that for the half drunk Thoros, it is crystal clear, that he merely prayed and the Lord of Light did the work. Still, Melisandre seems to know that resurrection is something the priest does/did. To me it's clear, that she is more into everything concerning the Lord of Light than Thoros has ever been.

Could it be, that Melisandre is actually a witch who is very aware of it and the nature of her powers and merely uses the religion of the Lord of Light to train, explain and (re-)establish magic in the world, but doesn't actually believe in R'hllor?

Answer 1

Firstly, let's address the "witch" issue. Melisandre certainly does wield magic. Her power is not just given from R'hllor, it's something she studied (along with all of the other Red Priests). Their belief is that the magic comes from R'hllor, as a gift to them, but they must still learn to wield it. So, depending on your definition, she is a "witch", whether she believes in R'hllor or not.

Why did she seem surprised that Thoros brought back Beric Dondarrion? Well Thoros was never particular pious, so he wouldn't have received many powers from R'hllor. This doesn't really bring her faith in R'hllor into question, just her faith in Thoros, which is understandable given his past.

What does suggest that she may just be a witch? We know that Melisandre is a Shadowbinder. We don't know much about them but the name would suggest that the power of creating a shadow comes from her other magic, not from R'hllor. Melisandre claims that these shadows are servants of light. Whether she believes this or not, is impossible to say.

There's nothing in the books to suggest she doesn't believe in R'hllor, so any answer to the contrary can only be hypothesis and theory.

Here's my personal theory on the R'hllor situation (spoilered as it's based across the whole series):

Stannis isn't the chosen one of R'hllor. There's evidence to support this. It's fairly well established that his sword isn't actually Lightbringer. Salador Saan says to Davos, "that sword was not Lightbringer, my friend", and Maester Aemon says to Samwell, "The sword is wrong, she has to know that ... light without heat ... an empty glamour ... the sword is wrong, and the false light can only lead us deeper into darkness." Lightbringer is said to be a sword made of living fire. The only time we've seen this is with Lord Beric Dondarrion, who has a sword that can become engulfed in flames. Couple this with the fact that Thoros keeps bringing him back to life, and I believe that the Lord Beric is the true chosen one of R'hllor. This would explain why Melisandre was so shocked. It didn't make her question her faith in R'hllor, it made her question her faith in Stannis.

Answer 2

Straight From the Horse's Mouth
There is a single chapter in A Dance With Dragons which takes Melisandre's view-point; in which she refers to Rh'llor many times as "Lord" and seems extremely devout in her belief.

Here are some relevant passages from the chapter:

Surely R’hllor would vouchsafe her a glimpse of what awaited him. Show me Stannis, Lord, she prayed. Show me your king, your instrument.
...
It made no matter. Melisandre of Asshai did not fear for herself. R’hllor would protect her.
...
Dawn. Another day is given us, R’hllor be praised.
...
Food. Yes, I should eat. Some days she forgot. R’hllor provided her with all the nourishment her body needed, but that was something best concealed from mortal men.
^{-A Song of Ice and Fire - A Dance With Dragons Part One: Steel and Snow, Chapter Thirty (Melisandre).}

It is important to point out that this is in the privacy of her own chambers - a place where she doesn't need to pretend to believe in anyone.

More Than a Believer
Melisandre is a strange (for lack of a better word) character. It seems evident that she is more than just someone who believes in Rh'llor, rather a champion of his!

George R. R. Martin points out in a recent interview that she is essentially operating on her own volition with Stannis:

[Interviewer] Why did Melisandre seek out Stannis? Did she see him in her flames and decided to seek him out on her own, or is she on a mission on behalf of the red priests? It doesn't seem at any point as if the latter is the case, when you compare to Moqorro who has been sent out by the priesthood.
[GRRM] You're right. Melisandre has gone to Stannis entirely on her own, and has her own agenda.

Answer 3

The over-arching theme in the build-up phase of A Song of Ice and Fire is that the magic is beginning to return to the world, right as all hell is about to bust loose.

In the preparations for the Battle of the Blackwater, Tyrion as acting Hand of the King commissions the production and training on the use of wild fire. When speaking with Hallyne the pyromancer, head of the Alchemists' Guild, Tyrion is asked if any dragons had been found. There was evidently an association between the dragons and ability of the pyromancers ease of making wildfire which had waned in the declining years since magic had left the world. As the production had grown easier, Hallyne was curious if Tyrion had heard anything. The process of creating of Valyrian steel is also believed to be extinct, though there are references to master iron workers being able to reforge existing Valyrian steel.

It's not a far stretch to put it all together; the red comet, coming of the new dragons, wildfire and magic, along with the impending winter in which the others would make their return. These are all foreshadowing of an epic war, waged between the wielders of fire and light against the frozen darkness. All part of the greater dichotomy.

So, Melisandre definitely believes in her powers being endowed by the will of R'hllor, even if the non-believers can influence magic. While she may not find Thoros to be a worthy disciple, her own confirmation bias would undoubtedly point to the faith in R'hllor being a component. As for her own work and purpose, as Andres F. has already well pointed out,

the use of Stannis as a false "chosen one" may only be indicative of her mis-interpretation of R'hllor's will.

Melisandre is devout. Of that, there can be no doubt. Everything she has done is in service to the Lord of Light; regardless of her interpretations.