Well, I suppose John's overt goal is to undergo his journey that he needs to complete in order to become a priest. However, at the same time, we are told that John's ultimate goal is to learn more to combat his ignorance on so many subjects concerning his people and their origin. Note the following quote:
Nevertheless, my knowledge and my lack of knowledge burned in me--I wished to know more. When I was a man at last, I came to my fahter and said, "It is time for me to go on my journey. Give me your leave."
John thus desires to find out more information and gain more knowledge, which he does in the end gain, but only after suffering many hardships and enduring many dangers. The vision that he experiences whilst in the skyscraper, where he views the end of our civilisation, teaches him the dangers of desiring too much knowledge, and he returns to his tribe and father a humbled and wiser individual, recognising that, "perhaps, in those days, they ate knowledge too fast."