I see the destruction of 19th century romaticism, the destruction of the rural lifestyle, and the evil of runaway industrialization in that tale.
Those elves represent the thinkers of the classic world, rennaisance, and 19th cent. romanticism.
The immortality of the grey havens represents the fame of great people—thinkers or doers—after they die.
The departing of the elves represents the change from a more pastoral life and way of thinking to the dog-eat-dog industrial twentieth century.
Gandolf represents science used rationally.
The bad wizard represents runaway industrial science (shown by the destruction of natural beauty, the forests, to fuel and be replaced by those underground factories.
The “resurrection” of Gandolf represents how one can be changed by the experience of war into someone of great inner strength, with a matured view of the meaning of life and death. It also reminded me of those people who come back to life after having almost died, and who remember death as so pleasant an experience that they no longer fear dying (expressed by Gandolf in his White Shores talk to the hobbit).
When Aragon enlists the dead in that mountain, that represents how those who accept their own death in a struggle (represented here by war), are those most likely to survive that struggle—they, like Aragon, take death as an ally.
The red glow and various lights of Mordor on the horizon represent the explosions of distant modern warfare on the horizon.
The Ents represent how the natural world can destroy those who carelessly damage it through rampant industrialization (as global warming today threatens our industrial lifestyle?).
I could tell more if I had the time.
In a Christian context I thought the Ents may have represented some form of good territorial principality.