One probable explanation for the Holy Grail is that its a later "invention" during, in the dark ages, circa 800 ad, during the christianization of the celts, the monks took part of their religion and adapted it to fit in the christian belief. The cup that its always filled with wine and the invincible lance are of cleary celtic origin. (But, beside that , there are others that have a material existence, like the lance that the crusades took from Antioquia during the pillage and the Carlomagno's lance, every one supposed to be the Holy Lance, or the Destiny's Stone, recently moved from England to Scotland).
But the myth of the Holy Grail that we know nowadays is based on the King Arthur's legends. And it have little to do with the Templars.