The negative and all-embracing concept of "paganism" is really the shadow each kind of monotheism, or "revealed religion", casts on everything else, including its own past.
When Josiah decided that only Yhwh could be worshipped only in the Jerusalem temple, all the old sanctuaries of Canaan-Israel instantaneously and retrospectively became "pagan".
When the monotheistic exilees came back from Babylon to Jerusalem to rebuild the temple, all the "people of the land" who had not been exiled and who went on worshipping in the old Israelite fashion became "pagan".
When Christianity became the state religion of the Roman empire, all the traditional religions and rituals everywhere became "pagan".
When Islam came up, Christianity became "pagan" to the Moslems.
When Protestantism came up, most of the Catholic religion became "pagan" to the Protestants.
When the JWs came up, all Christendom became "pagan" to them.
See how it works?