In "By the Waters of Babylon" who are the forest people that John is so afraid of?
The world that John lives in is much different from our modern civilisation; in fact, it resembles ancient tribal communities much more, where people grouped together in families, and grew from there, forming tribes. Maybe compare it to Native American tribes, like the Cherokee and Sioux. And, many Native American tribes fought with each other; they were enemies fighting for resources, territory, and over cultural differences and blood feuds.
So, in John's society, the "Forest People" are just another tribe, who emphasize different things, who live in a different area, and who have different customs than John's people do. He mentions that the Forest People are "ignorant," which, according to him, means that they do not know how to read the "ancient writings" and they don't spin wool like his people do. Instead, they "eat grubs." Later, on his journey, he mentions how he passes by an entire "hunting party" of Forest People, without them even seeing him, which means that he is blessed on his travels. Later, he says that as he sleeps,
"The Forest People could have killed me without fight, if they had come upon me then, but they did not come."
So obviously the Forest People pose a dangerous threat--they would most likely kill John if they saw him. This alludes to a feud between the tribes, and animosity and violence between them. So, he is afraid of them at first because they probably hunt down and kill anyone belonging to his tribe. But, he feels protected on his journey, and escapes any encounters with these dangerous people. I hope that those thoughts help; good luck!