Guenever is overdressed for the occasion—her face is too painted. Lancelot however sees the same girl of twenty, trying desperately to defy the doom of human destiny. Lancelot tells Arthur and Guenever about his altered state of being—he has become more devout and spiritual.
Lancelot's quest—although it did not culminate in him finding the Holy Grail—has led to a reformation of his character.
He had started his Quest by travelling to King Peles' castle, but was waylaid on the journey when he was dismounted by Galahad. Angry and with his pride hurt, he had ridden to a chapel. While he slept, he dreamt a knight came and took all his armor and weapons and, when he woke, all his knightly things were gone. He confessed the biggest sin on his conscience (Lancelot seems about to spill the secret of his relationship with Guenever, but she stops him).
The sin Lancelot confesses is of his relationship with Guenever. When he makes as if to tell Arthur—this would have made his purification complete—Guenever interrupts and does not let him. She is not ready to face that sin, and his love for her stops him from confessing it for both of them.
Lancelot was given penance and rode away. He came across two warring sides in a tournament and decided to join the losing side to rescue them. However, still they lost and he was taken prisoner, beaten and disgraced. He realized he had done penance for one sin, but not for another—the pride that makes him show off and compete for the losing side. He confessed again. Next he rode onwards and was knocked down by a black knight in defeat again. Although he had been truly absolved, God was not going to give him back his vice simply because he had confessed.
The second sin Lancelot must confess to is the very sin Arthur has been trying to purify his realm of—the act of doing good has become simply another form of knightly egotism instead of an act of justice.