In Sir Gawain and the Green Knight, Gawain's quest is the result of taking the Green Knight's challenge. At Arthur's court at Christmas, the Green Knight says he will allow anyone to cut off his head if that man will return the favor. Gawain is sure that cutting off the Green Knight's head will kill him, so he won't be able to kill Gawain!
The theme here is that things are not always as they appear. However, Gawain is inexperienced and he takes his swing; the enchanted green warrior picks up his head, tucks it under his arm, and reminds Gawain to meet him in a year's time to honor the bargain they made.
Gawain feels sure that he will die. However, he is an honorable knight of Arthur's Round Table. He starts out the following winter in time to meet the Green Knight as arranged.
The story is indeed about human sin (telling a lie) and redemption (being forgiven). However, the story also takes into account Gawain's humanity. For while he returns Lady Bertilak's kisses to Lord Bertilak, he conceals he magic belt that will save his life. This desire to cheat death causes Gawain's sin of omission—when he fails to share the knowledge of this gift with his host.
Gawain is successful in his quest: for he faces the Green Knight as he is honor-bound to do.
By God...may the Holy Ghost
Grant me the power to begrudge you nothing.
Keep to the bargain, swing just once,
And I'll stand still...
Gawain takes up his position, head bent and neck exposed, but "his knees / Were weak." His fear of death is evident when he flinches as the giant takes his "ax stroke"—stopping his swing just short of Gawain's neck. The Green Knight chastises him, calling his bravery and honor into question.
You can't be Gawain...I've heard nothing of Gawain the coward.
Gawain admits his fault and prepares for a second assault, noting:
I won't [flinch] again.
And this much is plain:
My head, if it falls, won't talk in my hands.
Gawain faces the certainty of his death—no magic will spare him. But Gawain is disgusted with himself, and in this he learns humility. He demands the Green Knight hurry to his task. As he promised, Gawain does not move as the blade falls, but still the Green Knight stops himself from killing the younger man.
Gawain is furious; he insults the green man, hoping to force him to finish. The green warrior takes a swing, but intentionally only nicks Gawain, who swiftly rises and assumes a battle stance, for he has paid his debt. The Green Knight approves of this young knight, who he has been testing. He explains that he stopped the ax the first time because Gawain was honest about the kisses, but he notes that Gawain felt his sin in neglecting to share the gift of the belt. This is tangible evidence of Gawain's failure—love of life over honor. The knight notes that Gawain acted simply "for love of your life," which he understands, but Gawain is humiliated:
Fear of your blow taught me cowardice.
Gawain asks for forgiveness; the knight (like God, who Gawain serves) grants it immediately—telling him his soul is a pure a snow, without sin. He gives Gawain the belt as a memento. However, Gawain learns humility, and the true call of chivalry: he understands his shortcomings, and palns to dedicate himself to overcoming them.
His failure is not overstated: for this is the core of his chivalric beliefs, which guides every step of his life.
Adventures in English Literature, Harcourt Brace Jovanovich Publishers: Orlando, 1985.