In "Sir Gawain and the Green Knight," everything that takes place after the green giant enters the hall at Camelot rests upon the importance of an oath.
The Green Knight appears—giving a challenge:
If any knight be so bold as to prove my words let him come swiftly to me here, and take this weapon, I quit claim to it, he may keep it as his own, and I will abide his stroke, firm on the floor. Then shalt thou give me the right to deal him another, the respite of a year and a day shall he have.
Gawain takes it up the challenge. The Green Knight asks for his promise that he will let the Green Knight have a "swing" at Gawain in return.
Then the Green Knight spake to Sir Gawain, "Make we our covenant ere we go further."
Gawain agrees. However, the knight is enchanted and picks up his head after Gawain removes it, reminding the Gawain of his promise, so he is honor-bound to act in accordance with his oath. In a year's time, Gawain travels to meet the Green Knight and keep his word.
When Gawain reaches the area where the Green Knight lives, he comes to a castle and is invited to spend the holiday with a lord—Bertilak—and his wife.
Then they questioned that prince courteously of whence he came; and he told them that he was of the court of Arthur, who is the rich royal King of the Round Table, and that it was Gawain himself who was within their walls, and would keep Christmas with them...And when the lord of the castle heard those tidings he laughed aloud for gladness...
Gawain visits and attends church services with the lord and lady of the castle. They celebrate the holiday together. As Bertilak prepares to go hunting, he tells Gawain to stay behind and rest. However, he offers an agreement to Gawain—whatever Bertilak wins during the hunt, he will share it with Gawain—so whatever Gawain "comes by" each day, he will share it with Bertilak. In agreeing, Gawain makes another oath.
In addition, as a knight Gawain has also sworn an oath of chivalry:
...usually associated with ideals of knightly virtues, honor and courtly love.
Chivalry was something that was created "guide" knightly behavior:
Christianity had a modifying influence onthe...concept of heroism and virtue...with limits placed on knights to protect and honor the [weak] and...help the church maintain peace.
This kind of warrior was sometimes known as "a knight of Christ," and piety was one of his virtues; often such men would take an oath of allegiance to the Virgin Mary.
Along with his oath to King Arthur, Gawain also takes an oath to the Green Knight, may well be honor-bound to serve the Holy Church, and has now entered into an agreement with Bertilak.
Each day when his host returns with a kill, he shares it with Gawain—who shares with the lord kisses given by the Lady Bertilak. However, though she attempts to seduuce him, Gawain is always courteous with his hostess—but he does keep secret a magic belt she gives him to protect him from harm.
At the appointed time, Gawain goes to meet the Green Knight: it is actually Bertilak. He knows of his wife's kisses and of the belt. Gawain is ashamed of his fear and deceit over the belt. The Green Knight swings at Gawain's neck and the knight flinches; this also shames the young man. Gawain takes two more swings by the giant man, who finally only nicks Gawain—and so the oath between them is fulfilled; Bertilak even forgives Gawain's deception.
The story of Gawain and his adventures rest upon the making and keeping of oaths.