When St. Paul went to Greece (Acts 17), he stood up in the Areopagus of Athens next to the altar they had placed there, which said “To an Unknown God,” and he tried to explain Christianity to a crowd of Greeks by saying that he knew their “Unknown God.” Even though this was only technically true, like Iseult’s half-truth about having been in the arms of no man but her husband’s and the beggar’s (ahem-cough-Tristan-cough), it got Paul far enough into the trust of the crowd that some of them wanted to hear him again on the subject (not that they blindly believed, which says nothing for their intelligence).
I have a sneaky suspicion that the people who “cleverly” inserted the Christian traditions and symbolism into the story of the Grail were taking a leaf out of Paul’s book (pun intended). By using the symbolism that was already established, the missionaries would be able to explain Christianity in terms that the people understood. After all, the symbols that were in use already did have some connection to Christianity.
update: I deleted the rest of the post. It is just too long. If you want to know more, I can send it to you, but . . . right. That was a lot of stuff to read through, and not the best written, either. Tada.