Thomas Alfred Spalding 
Elizabethan Demonology [EPUB ebook] 

The last and most important of these principles is the tendency of all theological systems to absorb into themselves the deities extraneous to themselves, not as gods, but as inferior, or even evil, spirits. The actual existence of the foreign deity is not for a moment disputed, the presumption in favour of innumerable spiritual agencies being far too strong to allow the possibility of such a doubt; but just as the alien is looked upon as an inferior being, created chiefly for the use and benefit of the chosen people—and what nation is not, if its opinion of itself may be relied upon, a chosen people?—so the god the alien worships is a spirit of inferior power and capacity, and can be recognized solely as occupying a position subordinate to that of the gods of the land.
This principle has such an important influence in the elaboration of the belief in demons, that it is worth while to illustrate the generality of its application.