Charles Williams argued in The Figure of Arthur that the Grail Quest was the backbone, the whole point, of the Matter of Britain. The Holy Grail is a symbol of divine grace, as administered through the sacrament of communion. The Grail contains the true blood of Christ and a vision of it is given to the Round Table at Pentecost. All of them then set out to experience it more fully. This becomes a test for them: if they pass the test, they will come at last to the Grail and be allowed to take communion from it. Only the holy will attain it, and three knights do find it: Sir Bors, Sir Percival, and Sir Launcelot's son Sir Galahad, the Grail Knight.
I suppose I always assumed that the Grail Quest was achieved. That it was a happy ending. Sometimes I would wonder how, if that was the case, Logres failed and fell anyway. But I didn't understand till I read Malory what was actually going on in the Grail Quest.
The Quest wasn't just for three knights; it was for all of them. The Quest was for the whole Round Table. And the Round Table failed. That was why it fell.
Sir Percival is one of the three knights who does achieve the Quest. Of him Malory says:
And as the tale telleth, he was one of the men of the world at that time which most believed in Our Lord Jesu Christ, for in those days there were but few folks that believed in God perfectly. For in those days the son spared not the father no more than a stranger.
A few pages later, Sir Percival explains himself as follows:
“Damosel,” said Sir Percival, “I serve the best man of the world, and in his service he will not suffer me to die, for who that knocketh shall enter, and who that asketh shall have, and who seeketh him he hideth him not.”
These short passages, especially the first, go a long way to explaining the Morte. Sir Percival, who is pure, courteous, and a true knight to his Lord, is an anomaly. As Sir Gawain's vision of the bulls shows (Book XVI, chapters 1 and 3), the three knights of the Grail Quest are the only knights of the Table who remain faithful to its original purpose and to the oath of the Table. The Round Table is not supposed to be a picture of virtue at all.
And suddenly, everything falls into place. This is no nostalgic picture of righteousness. This is no one brief, shining moment called Camelot. It is one of the greatest literary visions of sic transit gloria mundi that I have ever come across. The picture is one of men called to high things, of their failure, and of their destruction as a result of their failure.
In this sense, Sir Launcelot is the embodiment of the Round Table. Like them, he represents the best of the secular world—the peak of its glory. Like them, it is from him that the Grail Knight comes: only the best of the secular world is good enough to enter the sacred world. Like them, he comes close to attaining the Grail, but loses it in the end.
Of course, it is only from the best of the secular world that the Grail Knights can come. But the Round Table as a body falls short of the sacred world. That is why it is destroyed. CS Lewis confirms this conclusion in Studies in Medieval and Renaissance Literature:
The human tragedy becomes all the more impressive if we see it against the background of the Grail, and the failure of the Quest becomes all the more impressive if it is felt thus reverberating through all the human relationships of the Arthurian world. No one wants the Grail to overthrow the Round Table directly, by a fiat of spiritual magic. What we want is to see the Round Table sibi relictus, falling back from the peak that failed to reach heaven and so abandoned to those tendencies within it which must work its destruction. And that is what we are shown.
And now I realised why I had been so confused and irritated by Malory before. I had the story upside-down. I thought it was about the noble, pure, and good Round Table going about righting wrongs; so why were the knights always doing the wrong thing? Malory was clearly trying to write a morality tale; so why was everyone so immoral? At last I realised that the story really does have a moral compass--it's just that not everyone who I thought was meant to be a hero, is. The knights are bad because they're meant to be. The Round Table is a glorious failure.
There's an immense, cohesive story going on underneath the Morte D'Arthur. From the first seeds of Camelot's fall, sown in Arthur's youth, to the gradual degeneration of the Table, the failure of the Grail Quest, and the final stages where the Table—and the marriage of Arthur and Guenever--becomes too rotten to hold together, the tragedy is guaranteed.
That is the meaning of Le Morte D'Arthur.