The Perlesvaus — also known as Le Haut Livre du Graal ("The High Book of the Grail"), an early 13th-century Old French prose romance. It opens with Arthur making a pilgrimage to the Chapel of Saint Augustine (Saint Austin) in Wales, where he has a vision of Christ as a child and as redeemer, and experiences a kind of spiritual regeneration at a mass conducted by a hermit.
King Arthur rode into the forest and came upon the Chapel of Saint Augustine, where a hermit was about to sing the mass. The king tied his horse to a tree and decided to enter the Chapel. But, to his mortification, he was unable to enter, although there was no one there to stop him and the door was open. The king looked at the hermit and saw at his right hand the fairest child he had ever seen; he was dressed in a white robe and wore a golden crown laden with precious stones that shone in the light. On the hermit’s left side was a lady so fair that her beauty was beyond compare. When the holy hermit approached the altar, the lady took her son and went to sit on a jeweled chair to the right of the altar. She placed her son upon her knees and, kissing him sweetly, said, “Sir, you are my father and my son and my Lord, and guardian of me and all of the world.” King Arthur marvelled that she should call the child her father and her son. At the moment the mass was begun, the king saw a light come through the window and shine upon the altar; it was brighter than any ray from the sun, moon, or stars. The king was angered that he could not enter the Chapel to hear the hermit sing the mass and the beautiful responses that seemed like the singing of angels. When the Holy Gospel was read, the lady took her child and offered him into the hands of the hermit. The hermit set the child upon the altar and began the sacrament. King Arthur went down on his knees and began to pray. As he looked toward the altar, it seemed that the hermit held between his hands a man who was bleeding from his side and in his palms and feet, and was crowned with thorns. - The Gifts of the Grail by John Matthews, translating/retelling is Branch I of the Perlesvaus
Arthur arrives at the Chapel of St. Augustine but is physically barred from entering by a magical barrier — inside, Mary and Jesus are present at mass with a hermit. Once mass concludes, they disappear and Arthur can enter. The hermit, who had known Arthur's father Uther, then exhorts Arthur to mend his ways and informs him of Perlesvaus's disastrous failure at the Fisher King's castle.
The vision itself — Mary holding the Christ child and calling him "my father and my son and my Lord," then the hermit appearing to hold a bleeding, crucified man — is one of the most theologically striking scenes in all the Grail literature. The boy seems to transform briefly into Jesus before turning back into the boy, and the scene is filled with such holiness that Arthur is unable to enter the chapel until they leave.
This scene is one of the clearest examples in Grail literature of the Eucharistic vision as theophany — the mass as initiatory encounter with the Logos made flesh. The "barred from entering" motif is significant: Arthur's spiritual unworthiness creates an invisible threshold. The chapel is permeable only to those in a state of grace. The hermit's role here is also classic — guardian of the threshold, bearer of the gnosis Arthur needs to begin his restoration.
The Perlesvaus itself is freely available online in the Sebastian Evans translation (1898) as The High History of the Holy Grail. That's the most accessible English version, though somewhat archaic. Bryant's 1978 Penguin translation (Perlesvaus: The High Book of the Grail) is more readable and closer to modern scholarship.