That King Arthur got mixed up with Jesus Christ can’t be too surprising, given the myth-making that went into the King Arthur story. Rumor has it that Bran, for whom the castle, Dinas Bran, was named, was Joseph of Arimithea’s son-in-law. Legend has it that after Jesus’ death, Joseph brought the Cup of Christ from Israel to Britain. It does seem unlikely, doesn’t it?
But that is what the ‘Holy Grail’ is, that King Arthur’s knights go in search of: “The Holy Grail of Christian legend is the vessel given by Christ to his disciples to sup from at the Last Supper. Later, it is said to have been given to his grand-uncle, St. Joseph of Arimathea, who used to collect Christ’s blood and sweat whilst he hung upon the Cross.” http://www.arthurianadventure.com/holy_grail.htm
Dinas Bran, in turn, is the “site of an ancient Iron-Age hill-fort, believed to have been the home of the Kings of Powys, well into the 8th century. It is particularly associated with King Elisedd of Eliseg’s Pillar fame. The castle is, however, named for King Bran Fendigaid (the Blessed), a Celtic God known from both Welsh and Irish mythology who was later mortalized into a monarch of North Wales.” http://www.britannia.com/history/arthur/corbenic.html
“Much of the information available about Bran the Blessed strongly suggests that at least part of his legend entered into later Arthurian romance. His Magic Cauldron is probably that sought by King Arthur in the Welsh poem, the “Spoils of the Annwfn”. As in Bran’s Irish tale, Arthur travels to the Celtic Otherworld and, like the Welsh tale, only seven men survive. The vessel was later reborn as the Holy Grail, the cup of plenty or cornucopia found in mythology from across the Globe. The wound to Bran’s foot, inflicted by a poisoned spear, which caused his lands to fail is echoed in that of the Arthurian Grail guardian, known as the Grail or Fisher King.
His latter title may be related to Bran’s association with rivers and river-crossings (such as those he encountered in Ireland). His castle was Corbenic or Castell Dinas Bran, both names deriving from the word Raven or Crow. The Fisher King, like Bran’s head, could feast with his followers indefinitely and his forename was said to be Bron (or Brons) in the so-called Didot Perceval: clearly a transformation of Bran. Here, he is given a wife, Anna, the daughter of St. St. Joseph of Arimathea, probably through confusion with his grandmother, Beli Mawr’s wife, Anu. Bran may also be the original of other Arthurian characters like Brandegorre, Bran de Lis, Brandelidelin or Ban of Benoic.” http://www.whiterosesgarden.com/Nature_of_Evil/Underworld/UNDR_Deities/UNDR-D_western_europe/UNDR_bran2.htm
It was Joseph of Arimathea who gave his tomb to Christ upon his death and (again, legend has it) first brought Christianity to Britain aroun 63 AD, along with the cup.
“During the late 12th century, Joseph became connected with the Arthurian cycle, appearing in them as the first keeper of the Holy Grail This idea first appears in Robert de Boron’s Joseph d’Arimathie, in which Joseph receives the Grail from an apparition of Jesus and sends it with his followers to Britain. This theme is elaborated upon in Boron’s sequels and in subsequent Arthurian works penned by others. Later retellings of the story contend that Joseph of Arimathea himself travelled to Britain and became the first Christian bishop in the Isles.” http://en.wikipedia.org/wiki/Joseph_of_Arimathea
Glastonbury Tor claims this too, but we know that can’t be true