It must be understood that in some cases the process by which a god or goddess degenerates into a fairy may occupy centuries, and that in the passage of generations such an alteration may be brought about in appearance and traits as to make it seem impossible that any relationship actually exists between the old form and the new. This may be accounted for by the circumstance that in gradually assuming the traits of fairyhood the god or goddess may also have taken on the characteristics of fairies which Already existed in the minds of the folk, the elves of a past age, who were already elves at a period when he or she still flourished in the full vigour of godhead. For in one sense Faerie represents a species of limbo, a great abyss of traditional material, into which every kind of ancient belief came to be cast as the acceptance of one new faith after another dictated the abandonment of forms and ideas unacceptable to its doctrines. The difference between god and fairy is indeed the difference between religion and folk-lore.
Lewis Spence, British Fairy Origins
fairies, fairy, folklore, gods, mythology