One thing that makes Tolkien's fantasy so real, and so fundamentally Christian, is the way he portrays power. Power in Tolkien's subcreation is not like the Force in Star Wars. The Force would be at home within a Manichean philosophy; it has light and dark sides that seem to run parallel to each other and to be of equal power. A person decides which side of the Force to use, but both light and dark sides are contained within the Force. The Force itself is amoral and impersonal, as is shown in the very terminology of using the Force. This type of power is not at all uncommon in fantasy literature (and Star Wars has much more in common with fantasy than it does with science fiction).
Tolkien, on the other hand, portrays power as something that's so personal that legitimate (legal, "allowable") power can come only from The One. In his letters, JRRT says that, in his subcreation, if power isn't connected to Eru either directly or through the Valar, it's an "ominous and sinister" word. Saruman and Sauron make their power sinister, as well as weaker, by breaking that connection. Gandalf, on the other hand, strengthens that connection and becomes "Saruman as he should have been." Gandalf returns home at the end of his work in Middle-earth because he hasn't tried to claim power for himself, but to serve and recognize the One from whom his power ultimately comes. Sauron and Saruman come to believe in and depend on their own power, and are each dissipated by a puff of wind when they turn toward what had been their home. If we look at power from this perspective, it's easy to see that Tolkien does not consider good and evil to be equally powerful.
Frodo aligns himself with legitimate power from the beginning. The fact that he spontaneously calls on Elbereth as early as Weathertop says to me that it's not a new idea for him, and was probably instilled in him by Bilbo. It's an Elvish spirituality, because that's who Bilbo learned it from, but that doesn't make it invalid for mortals (especially if they happen to have a bit of fairy blood). At the ford, the Witch-king's response to Frodo calling on "Elbereth and Lúthien the Fair" (parallels to an angel and a saint) is to strike the hobbit dumb so he can't use the names again.
Tolkien's belief, both in his subcreation and in our primary creation as he viewed it, was that there is legitimate power and illegitimate power. If someone has the right (given, at least ultimately, by the One) to use a certain power, it can be used legitimately, although even one with a right to use a particular power can still misuse it. If someone does not have the right to use a certain power, any attempt to use it will be wrong. Denethor "talked the talk," in telling Boromir that, because of the high lineage of Gondor's royalty, even ten thousand years would not be enough "...to make a steward a king, if the king returns not." But in his stated refusal to turn the kingdom over to Aragorn, he "walks the walk" not of a steward but of someone who believes he has the right to use power that belongs to the king alone.
Aragorn himself speaks in true humility when he tells Gimli, speaking of the palantir, "I am the lawful master of the Stone, and I had both the right and the strength to use it, or so I judged. The right cannot be doubted. The strength was enough - barely." A little later he adds, "...in the end I wrenched the Stone to my own will. That alone he will find hard to endure." Here we have a mortal taking control of the palantir from Sauron, something not even Saruman could do. Legitimate power comes ultimately from the power of Eru; illegitimate power attempts to stand on its own. A mortal with legitimate power on his side can do something a wizard using illegitimate power cannot.
In a number of places in his letters, Tolkien speaks of two powers that he considered illegitimate: magic and machines (or even "the machine"). In order to live in our current world, most of us have had to make more peace with machines than Tolkien ever did, as he watched them destroy much of the creation he'd loved since childhood. (The war isn't completely lost, though. In one of his letters, Tolkien responds vehemently to the idea that building highways through Oxford is a good idea. On my visit there in August 2006, a tour guide proudly told us that many of the streets are now being turned into pedestrian-only pathways. I can't help but think Tolkien had something to do with that decision: "From your mouth to God's ear.")
But what about magic? In Tolkien's cosmos, aren't there legitimate - and, therefore, good - uses of magic? It depends on how the word is defined. What Sam calls "Elf-magic" is actually normal for Elves, and is a legitimate part of the nature Ilúvatar has given them. Galadriel finds it difficult to see how the same word could be used for Elves acting out of their true nature as is used for "the deceits of the Enemy." One sign of magic as a mode of illegitimate power, is that it attempts to control others and their wills, something legitimate power will not do (which follows from the fact that all legitimate power comes from God, Who allows us to sin rather than control our free will). Control of others is what Sauron hopes to gain through the Ruling Ring; Gandalf says of Saruman, "He will not serve, only command."
When JRRT pairs "magic" with "machines" in his letters, however, he's usually referring not to his subcreation but to our own primary creation and the means used to gain illegitimate power within it. Although magic, in the strict sense of the word, isn't the only method some use to try to gain sinister power in our world, Tolkien considered evil supernatural power to be real - and a real danger to the unwary. Elrond's statement during the Council that "It is perilous to study too deeply the arts of the Enemy, for good or for ill," is one Tolkien would have agreed with. He thought it was dangerous for C.S. Lewis to write The Screwtape Letters, because it necessarily put Lewis in the position of trying to understand the minds of demons. Charles Williams's interest in magic was one of the things that kept JRRT from becoming a close friend of his; there's no evidence that Williams actually practiced magic, but much of his fiction indicates that he studied it. A novel centered around tarot cards, such as Williams's The Greater Trumps, was not something Tolkien would have written. I'm not taking sides here; Lewis and Williams both wrote profoundly Christian literature - including The Screwtape Letters and The Greater Trumps - but Tolkien didn't agree with them on everything.
Belief in the reality of supernatural evil makes discernment of spirits necessary, and we see signs of the gift in at least two characters: Frodo, with his statement that a servant of the Enemy would 'seem fairer and feel fouler,' than Strider, and Faramir, who denies that the sight of Boromir's boat was a work of the Enemy by saying, 'For his works fill the heart with loathing; but my heart was filled with grief and pity.' And Tolkien brings up the subject himself in a letter (#328) to a reader that was quoted in an earlier essay:
...If sanctity inhabits [a man's] work or as a pervading light illumines it then it does not come from him but through him. And neither of you would perceive it in these terms unless it was with you also. Otherwise you would see and feel nothing, or (if some other spirit was present) you would be filled with contempt, nausea, hatred. 'Leaves out of the elf-country, gah!' 'Lembas - dust and ashes, we don't eat that.'
It's ironic that Tolkien's belief in the reality of evil has led to the charge by some present-day fantasy authors that his fiction doesn't make evil "real" enough. Much of what we don't learn about Sauron's activities would be spelled out in graphic detail by many other authors. In my opinion, though, the sense of real evil hovering as a shadow just outside our sight is more frightening than stomach-turning details of fictional evil played out right before our eyes. Because Tolkien spends little space in LotR describing evil, he has the opportunity to, as one commentator said, "make the good interesting." In LotR, it's the imperfect and flawed "good guys" who hold our attention and invite us to take a closer look at their struggles, victories, and even failures. Instead of focusing on vivid depictions of evil, LotR gives us vivid glimpses of good. I believe this is one of the ultimately indefinable things that draw people who are searching for good, or for The Good, to Tolkien's writings.
In his book about Tolkien titled Author of the Century, Tom Shippey places him with such twentieth century authors and works as William Golding's Lord of the Flies, which treat the corruption of power as an unavoidable human trait. I disagree with this, as I believe Tolkien did believe in the existence of good power - legitimate power - even among human beings. If Tolkien's cosmos paralleled the one we see in Lord of the Flies, the hobbits left to their own devices in the Shire would be torturing and murdering each other*; instead, we learn during the Scouring of the Shire that no hobbit has ever purposely killed another. The Ring isn't evil simply because it bestows power. It's intrinsically evil because its power operates by controlling the free will of others, which Tolkien saw as, by definition, evil. It's illegitimate power, "ominous and sinister" because it is not from Eru.
The Ring's ability to prolong life (without actually bestowing more life) is also a power not from Eru. One of the most ominous lines in The Lord of the Rings, especially coming as it does at the very beginning, is the statement by Bilbo's neighbors about his perennial youth: 'It isn't natural, and trouble will come of it.' Those of us who've already read the book know that, indeed, it's not natural and a great deal of trouble will come of it, although probably not in the way the neighbors expect. In Tolkien's cosmos, something being unnatural - stepping outside its God-given way of being - is not a good thing. Hobbits weren't meant to live for 60 years without physical aging; the only way it can happen is for life to be 'stretched... like butter that has been scraped over too much bread.' The Ring-wraiths continue an earthly existence long after they should have left it behind. The power of the Ring might provide a kind of "immortality," but it's very different from the immortality Elves have in their very nature - the nature given to them by God.
*Another of Golding's books that Shippey mentions is The Inheritors, which (according to Shippey; I haven't read it) tells how our human ancestors ruthlessly killed off the peaceful Neanderthals, evidence that human power always corrupts. Interestingly, recent genetic studies have shown that at least some of our ancestors interbred with the Neanderthals, so there's a good chance they were absorbed more than they were violently destroyed.