Short answers, pulled from the story.
Tolkien said in his Letters that the tale's central theme is death and the desire to escape it, not power and dominion. He wrote that it is mainly concerned with Death and Immortality, and with escapes such as serial longevity and hoarding memory.
Critic David M. Miller argues the quest is reversed because Frodo Baggins is not seeking a treasure but hoping to destroy one, unlike traditional quest heroes. Tom Shippey calls the story an anti-quest, a tale of renunciation rather than acquisition.
Philosopher Peter Kreeft and Jean Chausse identify Gandalf, Frodo, and Aragorn as reflecting the Old Testament's threefold Messianic symbolism of prophet, priest, and king. Gandalf's death and return from Moria, Frodo's suffering on the way to Mount Doom, and Aragorn's return from the Paths of the Dead each echo elements of the Christ story.
Scholars such as Ralph Wood note the book has been criticized for lacking significant female characters, for omitting religion, and for appearing racist. Wood and the scholar Sandra Ballif Straubhaar argue Tolkien was consistently anti-racist and that characters like Galadriel and Éowyn are far from insignificant.
Tom Shippey writes that the book embodies Tolkien's belief that the word authenticates the thing, reflecting his work as a professional philologist. Tolkien invented languages such as Quenya and Sindarin and insisted on traditional word forms like dwarves and elvish rather than dwarfs and elfish.
The Ring represents the corrupting influence of power, especially over those already powerful, as Tom Shippey has written of Gandalf's warnings. Powerful characters including Gandalf, Elrond, Galadriel, Aragorn, and Faramir all refuse to take the Ring, believing it would overpower them.