Who were John's forefathers in "By the Waters of Babylon"?
You need to focus on the setting to gain the answer to this question. Consider the dystopian focus of this masterful short story - we are presented with a scene in the distant future, focusing on what earth is like after some kind of nuclear holocaust where man's technology and knowledge has been responsible for ending our way of life as we know it and has returned us to an stone-age level of civilisation. Of course, the first-person narration of the tale means that there is a gap in knowledge for us as readers and we only find out the true historical setting as the tale progresses. Therefore the forefathers of John are actually going to be our ancestors - who perhaps have witnessed the spectacular end of civilisation and have had to piece together some kind of life with the remnants of humanity. The tale of course is meant to present a warning to us now about the potential dangers of "eating knowledge too fast" so that we do not unwittingly re-enact the catastrophe referred to in the story.