In "By the Waters of Babylon" how does John become more mature because of his journey?
Before he left, John was an adolescent still bound by tradition, obedience to his tribe and father, and unsure of his capabilities. Along his journey, he gathers strength and courage as he fights off wild animals, bests the Forest tribes, and makes decisions on his own. He gains confidence, and feels okay in passing previously set boundaries. He takes things further than anyone else in his tribes has done before, and as a result, gains more knowledge than anyone else.
His journey also opens his eyes to a great truth, that the gods were men, just like them. This gives him incredible knowledge and power; then, when he gets home, upon the advice of his father, he tempers that knowledge with wisdom, choosing to dispense his knowledge a bit at a time, in order for his people to use each bit wisely. We him making a very wise and mature decision; a normal adolescent would come home bragging about all that he had seen. John makes the more responsible choice.
He left his tribe a boy who saught permission to do all that he did. He was naive and expectant, insecure and had a very limited perspective. He came back full of confidence, having passed numerous tests of strength and courage, and armed with knowledge that would change him and his people for the good.