What is the holy grail and is it relevant to Christian doctrine?
Classically, the Holy Grail is the actual chalice used for drinking the wine used at the Last Supper on the night before Jesus was killed. During the Middle Ages many people became fascinated with relics. These were physical objects which were supposedly from Christ, the apostles or one of the early Christian martyrs. Common relics were supposed bones or fingers or other body parts of one of the apostles or of important martyrs, as well as items of clothing supposedly worn by them. The creation of such relics became a massive industry with fake relics being manufactured for profit. The most famous of these faked relics is the Shroud of Turin. Mark Twain once ironically commented that there are so many supposed relics of the true cross that one could shingle an entire barn with such relics. This was not an exaggeration. The emphasis on relics is obviously not supported or encouraged in the Bible. Historically, it became a distraction from true spirituality. The relics became the object of veneration when our worship should be directed only to God. It brought the church into disrepute as it revealed the superstitious nature of Christianity as practiced, especially in the Middle Ages. Relics were also used as a political tool to give strength to the political or religious authority of the established mainstream Roman Catholic or Greek Orthodox churches.
The “search” for the Holy Grail is part of the legacy of this unfortunate emphasis on relics. Many have claimed to have the actual cup used at the Supper. Of course, it is impossible to confirm such a claim. Besides, surely there were many cups used at the Supper. Which of the cups is the one being claimed to be the true one? Is it the one Peter drank from or was it Jesus who drank from this particular cup? This is all a lot of silliness, of course.
Let us imagine that we actually had the particular cup that Jesus drank from (even though there would be no way to confirm it). What would be the significance of this cup? The answer is clear. It would have no significance at all. It would just be a cup. It is not the cup which matters. It is not even the wine in the cup which matters. What matters is not what the cup contains, but what the cup represents, which is the blood of Jesus. We do not remember the cup, but we remember the blood of Jesus which sanctifies us. The search for the Holy Grail is interesting from an historical perspective, I suppose, but it is of no importance whatsoever to a Christian.