When the Holy Grail arrives in Arthur's court in a beam of light, covered in white silk, it fills the entire room with wonderful smells and tasty foods, and everyone present appears more beautiful than they ever have before. Sounds like the ultimate party trick, right? Oh, but it's so much more. So wonderful is this brief taste of the Grail that Gawain immediately declares his intent to go seek it again, stopping only when he catches full sight of it. Wanting in on the action, all of Arthur's knights jump on the bandwagon and take off, too.
The idea of a cup that bestows favorite foods, smells, and sights upon passersby seems kind of weird to us. There has to be something else to it, right? Right indeed. As it turns the "Tale of the Sankgreall," the Grail actually symbolizes the presence of God.
To a medieval Christian, being in God's presence was pretty much the most wonderful thing you could imagine, and the way to express this wonder was through the most pleasurable things – like food, smells, and sights – you could possibly imagine. When Arthur's knights head off on the Grail Quest, then, they aren't just on a quest for some more of that yummy roasted boar. They're trying to achieve unity with God by being in his presence, represented by full sight of the Grail.
If the Grail represents unity with God, or God's presence, the quest itself represents the life of the Christian as he struggles toward that unity. On the quest, the knights face all kinds of difficult choices, like when Bors has to decide between saving his brother, Lionel, or a damsel in distress. They're faced with temptation, like when Percyvale almost succumbs to the charms of a sexy lady only to be pulled back from the brink at the very last minute. How they make these choices helps us see whether or not they're worthy of the Grail.
The knights who have the most success on the quest are those who make sacrifices for God, particularly sacrifices involving their bodies. Percyvale and Galahad remain life-long virgins, and Launcelot wears a hair shirt and eats only bread and water. Medieval Christians believed that these kinds of sacrifices were the quickest way to heaven, which is why they're so important for the knights on the Grail Quest.
The challenges and temptations the knights face on the Grail Quest represent the challenges and temptations in the life of an ordinary Christian; if they meet and withstand them then they, like the successful Grail Knights, will achieve full sight of the Grail, and unity with God.