The event that determines John's readiness to become a priest is when John doesn't die when he holds the metal that his father hands him.
Then my father came out with the metal — good, strong piece. He looked at me with both eyes but I had not run away. He gave me the metal to hold — I took it and did not die. So he knew that I was truly his son and would be a priest in my time.
Holding onto a piece of metal and not dying is not the only prerequisite to become a priest. The first thing that is necessary to become a priest is to be the son of a priest. John tells readers that he is the son of a priest in paragraph two. Being the son of a priest gives John the privilege of accompanying his father to the Dead Places to search for metal. Being the son of a priest is not good enough, though. John has to be brave enough to hold the metal in the first place. He tells readers that his brothers were too scared, so they will never become priests.
Once John is identified as a future priest, he receives special treatment and begins special training. He gets the best cuts of meat to eat, and he gets the best spot around the fire. John is also allowed to search for metal by himself. Additionally, John receives training in basic first aid.
l was taught how to stop the running of blood from a wound and many secrets.
Lastly, John is taught how to read and write.