How does the Lancelot-Grail cycle contrast Lancelot with Galahad?

In the Vulgate cycle, the Lancelot-Grail narrative employs a deliberate contrast between Lancelot and his son, Galahad, to distinguish between earthly and spiritual devotion.12

• Lancelot: Representing the pinnacle of worldly chivalry, Lancelot is defined by his prowess and his adulterous love for Queen Guinevere. This earthly passion, while a source of inspiration for his knightly deeds, ultimately prevents him from achieving the Holy Grail and contributes to the collapse of the Round Table.12
• Galahad: Introduced as the "pure knight," Galahad is the embodiment of spiritual fervour. His character was developed to represent a higher, theological ideal, with a genealogy tracing back to the House of David. Unlike his father, Galahad’s achievements are driven by divine love, allowing him to succeed in the quest where Lancelot fails.12

By positioning Galahad as the son of Lancelot, the cycle effectively juxtaposes the limitations of human, romantic love against the transcendent perfection of spiritual devotion.12