Christianity is an interesting religion due to all the ideas it has purloined from other beliefs. The biggest one being Mithra. The people who worship as Jesuss worshipped 2000 years ago still get about in the Middle East region, I forget what they are called but they might be based in Iraq. Of course what they opractice is nothing like modern christianity.
Evil is in man, it is all too easy to leave the burden on to mythological creature and claim well it was Rex/Devil/Harpies/God did it etc. Not forgetting that evil is arbitrary, and what you call evil another man calls good. There isn't a universal line here.
It has been stated that the exodus from Egypt was actually by a bunch of uppity Egyptians not happy with the status quo. There is a really good book that although is not about the creation of religions and Gods, does deal with it slightly, called Uriel's Machine.