The Holy Grail is not mentioned anywhere in the bible. The last supper mentions a "cup", and that's it. If you read the original bible text, there is no reference to a cup being used to catch Christs blood. The Holy Grail's abilities are purely mythical, and therefore vary depending on what myth you are reading.
Early Arthurian Legend has the grail just being "holy" because of it's association with ******, not because it had any powers. It later developed powers as the legend expanded.
We probably made some false assumptions. The grail, as it exists in Crusade, has only ONE power. The ability to heal. Including healing some aspects of old age. Not all, just the aspects of old age that would cause your death. The Knight that left the temple and died sometime afterwards died not from crossing the seal, but from stopping drinking from the grail. No longer healing the death aspects of old age, he died. The 3rd Knight lived hundreds of years longer simply because he continued to drink from the Grail. So, it continued to keep him from dying. Indy and Father stopped drinking from the Grail when they left the temple, so would age and die naturally. (healing is not undone, so his father doesn't become 'shot' again.) Once the grail was lost in the chasm, the Knight could no longer drink from it, and he would die shortly after that as well.
