I first read Tolkien's essay On Fairy Stories about 1975, and liked it a lot - except for the Epilogue, which made no sense to me. It still made no sense to me when I re-read the essay about three years ago.
But I can understand it now, having become a Christian in the meantime.
Indeed, I can now perceive that this is one of the most important things that Tolkien ever wrote - because it was apparently the point when he justified to himself his own longstanding desire to write Fantasy.
Epilogue to On Fairy Stories, by JRR Tolkien - excerpts with my notes in [square brackets].
"Probably every writer making a secondary world, a fantasy, every sub-creator, wishes in some measure to be a real maker, or hopes that he is drawing on reality: hopes that the peculiar quality of this secondary world (if not all the details) are derived from Reality, or are flowing into it.
"If he indeed achieves a quality that can fairly be described by the dictionary definition: “inner consistency of reality,” it is difficult to conceive how this can be, if the work does not in some way partake of reality.
"The peculiar quality of the “joy” in successful Fantasy can thus be explained as a sudden glimpse of the underlying reality or truth.
"It is not only a “consolation” for the sorrow of this world, but a satisfaction, and an answer to that question, “Is it true?”
"The answer to this question that I gave at first was (quite rightly): “If you have built your little world well, yes: it is true in that world.” That is enough for the artist (or the artist part of the artist).
"But in the “eucatastrophe” we see in a brief vision that the answer may be greater—it may be a far-off gleam or echo of evangelium in the real world." (...)
[Tolkien is saying that successful fantasy is that which generates the peculiar emotion of joy, from a 'turn' in the story that he terms the eucatastrophe (the 'good catastrophe'). He is saying that this joy comes from recognition of a truth, and that this truth is ultimately a human version of the good news of the Christian story.
[In other words, while successful Fantasy does indeed offer legitimate satisfactions such as 'recovery, escape, consolation' there is even more to Fantasy than this.
[Tolkien is stating that the reason for Fantasy's power to delight and inspire, is that it is a 'far-off gleam or echo' of Christian story - and therefore that this power comes ultimately from God - and not from the artist. It is divinely inspired - and not a product of craft or artistry.]
"I would venture to say that approaching the Christian Story from this direction, it has long been my feeling (a joyous feeling) that God redeemed the corrupt making-creatures, men, in a way fitting to this aspect, as to others, of their strange nature.
[Tolkien means that the Fantasy writer, he is talking of himself, has had the impulsion to write Fairy Stories implanted by God - or rather than God has used the desire (which is probably in origin a corrupt and prideful desire) for his own purposes.]
"The Gospels contain a fairy-story, or a story of a larger kind which embraces all the essence of fairy-stories. They contain many marvels—peculiarly artistic, beautiful, and moving: “mythical” in their perfect, self-contained significance; and among the marvels is the greatest and most complete conceivable eucatastrophe.
"But this story has entered History and the primary world; the desire and aspiration of sub-creation has been raised to the fulfillment of Creation.
[This is the key sentence. The 'desire and aspiration of sub-creation' which Tolkien himself experiences 'has been raised to the fulfillment of Creation' - in other words, Tolkien's deep longing to write Fairy Stories can contribute - in however small a scale - to God's plan for the world. Tolkien goes on to spell this out:]
"The Birth of Christ is the eucatastrophe of Man’s history. The Resurrection is the eucatastrophe of the story of the Incarnation. This story begins and ends in joy. It has pre-eminently the “inner consistency of reality.” (...)
"But in God’s kingdom the presence of the greatest does not depress the small. Redeemed Man is still man. Story, fantasy, still go on, and should go on.
"The Evangelium has not abrogated legends; it has hallowed them, especially the “happy ending.”
"The Christian has still to work, with mind as well as body, to suffer, hope, and die; but he may now perceive that all his bents and faculties have a purpose, which can be redeemed.
"So great is the bounty with which he has been treated that he may now, perhaps, fairly dare to guess that in Fantasy he may actually assist in the effoliation and multiple enrichment of creation.
[This is, of course, precisely what the character Niggle does in the story Leaf by Niggle which - in most editions - accompanies the essay On Fairy Stories. Niggle's detailed picture of a many-leaved tree and its surrounding environment (or rather, his imagined ideal for such a picture) becomes real in heaven; is indeed added-to heaven, and assists in the salvation of human souls.]
"All tales may come true; and yet, at the last, redeemed, they may be as like and as unlike the forms that we give them as Man, finally redeemed, will be like and unlike the fallen that we know."