John realizes that the gods are not gods at all. The gods were regular men and women like himself. They lived a long time ago, and they were part of a great society that had lots of lights, cars, technology, and knowledge.
"—I knew then that they had been men, neither gods nor demons."
John learns these details as he is searching through one of the apartments. He sees things (i.e. water faucets) that look familiar but no longer work. John eventually falls asleep and has a dream (vision) that shows him New York in all of its glitz and grandeur. He realizes that the gods were only people. The final confirmation was John finding the preserved body of a human in the apartment. It had been preserved because of the room's high heat and low humidity.
"Then I saw the dead god. He was sitting in his chair, by the window, in a room I had not entered before and, for the first moment, I thought that he was alive. Then I saw the skin on the back of his hand—it was like dry leather. The room was shut, hot and dry—no doubt that had kept him as he was."