When a culture converts to Christianity, what happens to the old gods? For a long time, anthropologists and folklorists have argued that the gods do not disappear but ‘shrink’, diminishing into fairies and nature spirits 
This ‘small gods theory’ may sometimes be true, but doesn’t really hold water for most European cultures. The signs are that fairy-like beings had been honoured alongside the old gods anyway; gods didn't turn into fairies - rather, pre-Christian godlings survived Christianisation
However, we do find the gods in different forms after Christianisation - diminished, perhaps, but also more threatening. Deprived of sacrifice, the gods become enraged with the human beings who deny them worship - and they are hungry
We find this belief among converted Incas, Mayans and Aztecs in the New World - but also in Europe. Ragana, the terrifying cannibalistic Baltic goddess of witchcraft, is a case in point
Before the conversion of Lithuania, Ragana seems to have been a fairly benign forest goddess. It seems it was the coming of Christianity that made her angry, and hungry for human flesh. Is this down to Christian demonisation, or pagan fear of a dishonoured goddess?
Ultimately, Christianisation (and subsequent secularisation) does rob the old gods of their power. But there is a long intervening period when the gods become *more* threatening, as they are angered by being usurped
