Malory: Holiness and the Holy Grail

In the Tale of the Sangreal, I found the idea of holiness to be of interest. The Christian ideals of the time are so different from my ways of thinking. I was perplexed as to why Sir Launcelot wasn't "worthy" to pursue the Holy Grail. It was said that he committed sin in his love for Queen Gwynevere, specifically for fighting for her rather than fighting for God. I find myself feeling bad for Sir Launcelot. He tries to repent and is still told that he is not worthy to even see the Holy Grail. Sir Galahad on the other hand is somehow this great holy figure. Sir Launcelot and his son serve to juxtapose each other. Although they are related, Sir Launcelot is earthly and Sir Galahad is holy. This point is once again emphasized with the fight between the white and black knights. Here Sir Launcelot fought for the black knights, which obviously symbolize sin, whereas the white knights symbolize purity. It comes as no surprise then that the white knights won the battle. Everyone keeps yelling at Sir Launcelot to repent, though it seems that he already is. What I'm wondering is why all these knights went on this quest that it seems only Sir Galahad is capable of completing, knowing full well that they will probably die somewhere along the way. Even if they don't die, they certainly won't be able to accomplish what they set out to do.