The metal collected by the priests symbolizes the fears and superstitions of John's people. It was believed by the members of his village that if one touched metal, he/she would die unless that person was destined to be a priest. The superstition has its origins from the war that destroyed New York City. Metal after the blast would have been radioactive, and so anyone touching tainted metal would probably get radiation poisoning and die. So the adage of "don't touch metal" was born from that. Over centuries, the fear stuck. At some point, someone must have picked up metal and NOT died, and so that person was deemed as having special powers. After that, only those who could hold metal and not die were priests.
The fact that John is travelling east in his journey is significant. East symbolizes the quest for knowledge, and that is what John was seeking. He wanted to know about the Place of the Gods. He sees an eagle fly east and determines that this is a good omen. Eagles also symbolize knowledge.
The broken buildings and statues in what used to be New York City symbolize man's self destruction. Technology got out of hand, and someone used it to destroy known civilization. It is an apocalyptic story about the end of times, but offers hope that civilization can be reborn. John refrains from telling the truth about the gods in order to protect his people from a similar fate.