I think then it did mean, any religion except Christianity ( as did heathen)
I think it was slightly less derogatory than heathen but not by much.
I don't know but it seems unlikely to me that the pagans in the past thought of themselves as Pagans.
I think our understanding and meaning of those terms today, means something else.
Paganism is a huge umbrella for a large variety of beliefs across many areas.
I'm prepared to be proved wrong, but what is a Pagan? Exactly? Other than some one who follows a variety of older beliefs?
Feel free to tell me 🌹 
Both "pagan" and "heathen" were, and are, only ever used in a derogatory fashion by Christians - "Pagan" came from "paganus" which meant someone who did not live in a city or town, a rustic or villager - "Heathen" was used in almost the same way as "paganus" and meant the people of the heath(s).
Heathery is, in modern Pagan usage, used to refer to those who follow the path of Asatru, the Norse Gods and Goddesses, Odin, Freya etc whose names, incidentally, are still used by Christians for the days of the week!
Paganism is, as you say, a word used as an umbrella to cover, in its widest possible sense, any and all religious beliefs that are not parts of Christianity, Judaism and Islam; although I am not sure that Hindus, Sikhs, Buddhists, and those of the Far East among others, would appreciate their inclusion in the Pagan grouping.
However, when it comes to Paganism accuracy and truth have never seemed to bother the Christians on here, they will deny the truth of anything that either Rhiannon or I say about our beliefs and we are the only pagans currently on this forum so what would we know about our own beliefs?
We need to be taught what our beliefs really are by a bunch who follow beliefs whose followers tried for ceturies to wipe our beliefs off the face of the earth!
On this Forum they are STILL trying.
