Sayteth TVTropes.org:
The Holy Grail is rather more complicated. Back in the late 1100s, there was an incomplete poem by Chretien de Troyes, whose contributions to Arthurian canon were action packed and unconcerned with spiritual matters, in which a young, naive, Welsh knight named Perceval meets the Fisher King. A grail appears as part of a larger and quite bizarre mystical procession and is referred simply as “a grail” with no holy context, apart from carrying a host wafer. Perceval fails in his quest by not asking the Fisher King what the hell’s going on (making this story the first ever Sierra adventure game).
Over subsequent centuries, the Holy Grail grew into the entire raison d’etre of the entire Arthurian Court, when originally the Grail Quest was so singularly dangerous that there was a special chair at the Round Table reserved for those who dared attempt it, called the Siege Perilous. By giving the knights a single sacred focus rather than having them stumbling around England falling ass backwards into quests, this transformation made the sprawling tangle of stories more coherent, and elevated the moral standing of the knights.
The Holy Grail itself also grew hugely in significance, in some cases taking on parts of various other magic hamper and cauldron myths, which created a mythological snarl whose origins modern scholars are nowhere close to deciphering (compare to the several lucid theories about the Sword in the Stone that have cropped up in modern scholarship). By the 13th century, in Parzifal by Eschenbach, Parzifal’s calling to the Grail Quest is explicitly a calling to a higher and better world than the normal quests of Arthur’s court. The text claims that the Grail itself was the stone the neutral angels of Heaven stayed in during the war against Lucifer. By the 15th century, Mallory has the Grail be so powerful that when Galahad (the most pure and dedicated of all the knights) succeeds on the Grail quest he instantly ascends to Heaven.
Sayeth Wiki:
In the early tales, Percival’s immaturity prevents him from fulfilling his destiny when he first encounters the Grail, and he must grow spiritually and mentally before he can locate it again. In later tellings the Grail is symbol of God’s grace, available to all but only fully realized by those who prepare themselves spiritually, like the saintly Galahad.
The hero must prove himself worthy to be in the Grail’s presence
*This means the quest itself as well as the challenges the hero will face, and finally the Question.
”The Grail: Quest for the Eternal” by John Matthews
*The quest: a spiritual goal representing inner wholeness, union with the divine, self-fulfillment
*The Grail is housed in a temple on top of a mountain, surrounded by water, and protected by obstacles for the heroes to overcome
*the Seige Perilous: a seat only a perfect knight can sit in (in most tellings, Galahad)
*The Cathars had the ”consalamentum”, a kiss used to transmit light from one to another.
*Main themes are sacrifice and redemption
*Men seek in vain until the object of their search seeks ‘them’ out, and only then reveals its fullest meaning
*laspis lapsus ex caelis, the stone fallen from heaven: one of the descriptions from Wolfram von Eschenbach
**the stone itself needs to be redeemed and healed
**pure stone, philosopher’s stone–life and knowledge