Have you noticed that the devil is not mentioned in the Old Testament? There is a serpent in Genesis 3 yes, but is it the devil? Nowhere does God call it that. In fact in Genesis 2 it says that God created all the host of heaven and the host of the earth - and everything was good. But the devil is not good? Now someone will bring in Revelation 12 where we are told by John that the serpent is the devil and he is cast out of heaven by Michael and his angels through the blood of the Lamb and the word of their testimony. But is this literal? Are we truly understanding this scripture? Does an Arch angel (which many believe Michael is) need the blood of the Lamb? And this war took place after the birth of the Messiah according to Revelation 12;1-5, then the 'devil' was only cast to the earth after the Messiah's resurrection (caught up to the throne of God in verse 5) and could not have been the serpent in Genesis 3. In Genesis 4:7 God warns Cain, not against the devil, but against sin. The devil does not feature as an enemy of Israel in the Old Testament at all and never as an enemy of God.
"Satan" (satan in Hebrew) is not the name of a person or being, it is not a proper noun. In fact "satan" in Hebrew is just a noun and means an adversary.Look at how it is used in Numbers 22;32. If you use a concordance and look at all the places that this Hebrew word is used in the Old Testament, it is used to refer to human beings (1 Kings 11:14, 23, 25), the angel of the Lord (Numbers 22;22)and even to God Himself (compare 1 Chronicles 21;1 and 2 Samuel 24:1).
In the book of Job, this Hebrew noun is "hasatan" which means "the adversary". What this actually means is that in the midst of the sons of God who presented themselves before God, there was an adversary.
In the 400 years between Malachi and Matthew something happened in the belief system of the Jewish people because all of a sudden in the New Testament there is this being called the devil which has no basis in the Old Testament, in the Torah or in the Jewish belief system of the Old Testament.
Where does this concept of a devil infiltrate the New Testament authors belief system? and Why?