At the end of Le Morte d'Arthur, we are left with a few characters still standing. First, Sir Bedivere "swooned" and implores the church hermit, who is in actuality the former Bishop of Canterbury, to help him with "fasting and prayers." Bedivere claims, "For from hence will I never go . . . all the days of my life here to pray [for King Arthur]." Other knights arrive to adopt the same habits as Bedivere. This implies that the fervent loyalty of Arthur's knights, or what's left of his knights, will live on forever, perhaps inspiring others to take up the same way of life.
Furthering Arthur's legacy, "some men say . . . that King Arthur is not dead" but has gone "into another place" by the will of Christ. Some men believe Arthur will come again and that "he shall win the holy cross." Because of this and his sacrifice on the field of battle, he is given saint-like or Jesus-like characteristics. Like Jesus, it is implied that his story will be told generation after generation.
As for Queen Guenevere, she "stole away" to a nunnery in Almesbury, and "great penance she took" to make up for her sins. Her guilt and sadness over Arthur's death cause her to become a more holy person.
Arthur's tomb reads "Hic iacet Arthurus, rex quodam reque futurus": "Here lies Arthur, the once and future king."
In short, Arthur caused many people to turn to religion. His life story will live on into future generations, making him both a "once and future king." His legacy, Malory implies, is similar to Christ's and will live eternally.