You mentioned above that the Grail was a "Christian" idea and I'm not sure that it is - I mean it is now, post facto - but Chrétien basically invented it for his Arthurian stories; maybe drawing on Welsh and Breton myth.
The first known appearance of the grail was in
Perceval, le Conte du Graal (by Chrétien, aka
The Story of the Grail, written somewhere between 1180 and 1191). In that poem, the grail itself is not portrayed as having any supernatural powers. It is merely a dish (not a cup, but more like a large, wide, flat bowl), carrying a single communion wafer. As written, if there is magic, it seems to be in the wafer, or in the piety of the one
1 who can eat the wafer and be sustained, not the grail. If there is a connection to Celtic myth in the poem, it is more strongly and directly in similarities between the Fisher King and Bran the Blessed. That invites the thought of the grail-as-cauldron, though there's not much explicit support for it in the text.
The Grail becomes a holy object when Robert de Boron writes
Joseph d'Arimathie something like a decade later. This is where the Holy Grail as the cup from the Last Supper seems to originate in the Matter of Britain.
The Vulgate Cycle (aka the Lancelot-Grail) is completed by 1235, and the Post-Vulgate Cycle by 1240. And these two, more than anything else, were the basis for Morte d'Arthur, which is the most well-known version of the Arthur/Grail myth, which is published in 1485.
But it only entered popular consciousness as a Christian artifact in the centuries after the Arthurian stories gained traction.
I am not sure we can really claim it took centuries after Chreitian to become "popular consciousness", if only because claiming to know what stories were being popularly told in, say, 1290 is a bit of a bold assertion, hey what?
What we do know is that authors
kept coming back to the Matter of Britain. You want to claim they came back to it, but is wasn't well known in the populace?
1. Depending on the version of the story you get, it is either the Fisher King himself, or his even-more crippled father.