Most people are taught that the demons Jesus and His apostles cast out are fallen angels. But there is no passage that actually says that [to my knowledge.]
This came to me today as I was thinking about Jude 6 And angels who did not keep their own domain, but abandoned their proper abode, He has kept in eternal bonds under darkness for the judgment of the great day
2nd Peter 2:4 is similar.
The issue here is that Satan and his Angels are cast out of heaven immediately after Christ's resurrection [Revelation 12:9]. If Jude and Peter claim that these angels were put into Hell, then they could not have been around tormenting people during Acts, and Paul would not be too concerned about their teaching people in 1st Timothy 4:1.
The issue here is that if demons are not fallen angels, then what are they, and where do they come from? Some might find this just an odd question of no real relevance but some would find it very relevant because many teach that God cannot create anything evil. [The Bible itself says no such thing, but people think it would go against God's goodness. This is the same reason people claim Satan had to be an angel at some point. Actually, it seems the above verses would suggest that Satan is not, in fact, a fallen angel, as he is not confined in hell at the moment.]
Perhaps demons are evil souls of Nephlim who died.
Anyone else have some ideas?