The grall, sacred spear, and so on are... Not part of the earliest tales. So no. There's no guarantee whatsoever that it is a Celtic myth. Later stories were ce

The grall, sacred spear, and so on are... Not part of the earliest tales. So no. There's no guarantee whatsoever that it is a Celtic myth. Later stories were certainly modified by it. The graal first appears in 1190CE, "Perceval ou le Conte du Graal". Whereas Arthur appears in "Annales Cambriae" - maybe added to the text around 970CE, and probably earlier.