In "By the waters of Babylon," what does John's discovery about the gods make him realize?

Obviously, John's discovery makes him revise his religious beliefs. The ancients were neither "gods nor demons" but men. Religious edicts can no longer be justified on the basis of their divinity, and John clearly intends to violate some of the old laws. In the last paragraph of the story, he notes the city's name, and indicates his intention to take others into the city. In the first paragraph of the story, we're told that such actions have been "most strictly forbidden."
But John's realizations extend beyond the immediate theological implications. When he thought the ancients were gods, he regarded their achievements as beyond the reach of human beings. For instance, consider what he says about his dream—the dream where he witnesses a busy New York street at night:

"As I looked upon them and their magic, I felt like a child—but little more, it seemed to me, and they would pull down the moon from the sky.  I saw them with wisdom beyond wisdom and knowledge beyond knowledge. And yet not all they did was well done—even I could see that—their wisdom could not but grow until all was peace."

He perceived imperfection in the gods, but nevertheless believed they possessed "wisdom beyond wisdom." Their intellectual and technological feats appeared to be unattainable by mortals.
After his discovery, John indicates his intention to study the writings of the ancients and use this knowledge to advance society. ("We must build again.") He now believes it is possible to achieve some of the feats of the ancients. In fact, he even thinks it is possible to improve on their record—to follow in their footsteps without making the same mistakes. This is supported by John's analysis of what the ancients might have done wrong:

"[I]t is better that truth come little by little. I have learned that, being a priest. Perhaps, in the old days, they ate knowledge too fast."