Wherever you go, the experience is usually the same. You enter a church or a cathedral, and an ecclesiastical hush descends.

Around 200 cups, in Europe alone, are said to be the Holy Grail. But what makes one cup holy - but not another?

Pope Sixtus II entrusted the Holy Grail to Lawrence to save it from the persecution of Emperor Valerian. The deacon put the chalice in a safe place—and perhaps even sent it to Spain—before ...