When does John know is he is to be priest?
John lives in a futuristic society where much of the country has been destroyed in a "great burning" - presumably a war. In his society, there are "Dead Places" where most people are not allowed to go. As he explains, "The north and the west and the south are good hunting ground, but it is forbidden to go east. It is forbidden to go to any of the Dead Places except to search for metal and then he who touches the metal must be a priest or the son of a priest."
John is the son of a priest, and because of this he has been to the Dead Places near where he lives. At first he was afraid during these visits, but "it is not fitting that a priest's son should show fear" so he tries to be brave in his father's presence.
One day, when on such a trip with his father, his father returns with a strong piece of metal. The narrator bravely does not run away but instead takes the metal and holds it. He recalls, "I took it and did not die. So he knew that I was truly his son and would be a priest in my time."
This is the moment he knows he will be a priest. His brothers have not done this, even though they are good hunters. And after this, his father, too, knows he will be a priest and begins to treat him differently.