The Devil was originally called Satan. Satan was the Lord of the Earth and had access to Heaven. Satan was in paradise when Adam and Eve appear. Satan was the left hand man of God in the Old Testament. Satan is not thrown from Heaven until Revelations of the New Testament, which occurs after the time of Jesus. Then he becomes the Devil.
Satan in Heaven symbolized that the neural subroutine, due to dark side of law, was the driving force behind the conscious appearance of light; good side of law. In other words, if you lived in a dark evil place, like in a war zone, even subtle gestures of kindness, normally seen as evil during peace time, will appear as good, due to the contrast.
For example, instead of someone killing you with a sword, they kick you to unconsciousness, but you live and survive. This would seem merciful, under evil circumstances, even though it is still brutal. In that sense, the subroutines called Satan, induced by the original law, had the affect of creating a very dark contrast, so subtle light could be made conscious; better ways to behave. Gray can look white against pure black. The Satan Subroutine of dark side impulse, made people reflect on a kinder way, based on a contrast with this deep darkness, which eventually manifests as Jesus.
Jesus for-fills the law. Law created the polarization from which the Satan subroutine would induce a secondary subroutine; Jesus. The Jesus subroutine would replace Satan in the neural hierarchy. Jesus goes to heaven and Satan become more conscious or thrown to earth. It was a major update in the operating system of the human brain.
Satan thrown from Heaven to earth symbolizes the change in neural hierarchy with the Satan subroutine now connected to the shadow side of the conscious mind; ID of Freud. It becomes closer to the ego. There it is more subject to will power and conscious choice, instead of spontaneous actions; criminally insane. This new place for Satan is being reflected by the Jesus subroutine, which is very bright white; perfect. Subtle darkness now appears darker due to contrast with the bright light. Gray can look black if the white is very white. The brain is learning to become more differential.
A key invention for this evolution into the modern human mind, was the invention of writing. Before writing, there was only spoken language. Spoken language could also support law. However, with spoken language, law is not carved into stone. There is no easy way to reinforce the memory over the long term, so the inner polarization does not form to the same degree. Satan was originally Lucifer. Lucifer was a polarization subroutine due to spoken law, but it had much less potential. It was more of a creative rebel spirit. The invention of writing forced things to be carved into stone causing the polarization to become long term with much higher potential; death appears to contrast life and light.
The Devil is real in the sense it is like a computer program that forms in the human brain. The concept of original sin appears to indicate a mind over matter that have written this subroutine to genetics. It comes down to how much potential we consciously create. It can be Lucifer, Satan or the Devil.