The adjective paganus is derived from pagus and seems to originally mean roughly "belonging to a village". According to the L&S entry the sense "non-military" is also classically attested. In Christian use the word means "pagan". To what extent is this meaning a Christian invention? Did the adjective paganus mean any kind of non-believer or something similar enough before the advent of Christianity?
It could also be that the original Christian meaning of the adjective was different from how we understand "pagan" today, and the change occurred within Christian use. However, there seems to be a sharp contrast between the Christian meaning and the pre-Christian one, judging by the attestations I recall seeing. Perhaps intermediate examples of some kind would help bridge the gap between the two.
It is no reason for forgetting that the very word Pagan is the same as the word Peasant.I think he's got a good philosophical explanation for the change in the meaning of the word during after the Fall of Rome in chapter 8. I think the change in the meaning of the word may have taken place a few hundred years after Christ. Vulgar has a similar connotation. And the Latin Bible is called the vulgate, I guess it sounded better than Pagan Bible?