Dr. Heiser is an Old Testament scholar, author, and Semitic languages expert, and his work addresses the fact that satan in the OT is far different from Satan in the NT.
He writes, “In the Old Testament, the Hebrew word satan is not a proper personal name. This is because it is nearly always paired with the definite article in Hebrew (the word ‘the’). Like English, Hebrew does not permit the definite article to be paired with a proper personal name (I don’t call myself, ‘the Mike’). The common noun satan paired with the definite article means ‘the adversary’—not ‘Satan’ as in the proper name of the Devil. This is why some English Bibles have ‘the Adversary’ in passages like Job 1:6 and not ‘Satan.’”
Dr. Heiser points out the noun satan occurs 27 times in the Hebrew Bible and only lacks the article 10 times. Of those 10 times, seven refer to humans and three refer to the Angel of Yahweh.
He concludes, “there are ZERO verses in the OT that have a personal name “satan,” and ZERO references to Satan as a cosmic evil entity as in the NT.”
Old Testament scholar John Goldingay agrees, “The word for adversary/accuser is satan…The word is not a name but a common noun; it has the article the. The earliest otherwise-known occurrences of Satan as a proper name come in Jewish writings from the second century [B.C.]…” (italics his)
The book of Job is the oldest document in the Hebrew Bible and in this book we see that “the satan” is an officer of Yahweh’s divine council. He “accuses” anyone he catches not obeying Yahweh. He’s not an evil being opposed to Yahweh; he’s simply doing the job he’s been given. The story of Job should be viewed as a friendly bet and not as a contest between good and evil.
Thom Stark, who has written quite extensively on second-temple apocalyptic Judaism and Christian origins, writes, “a dualistic cosmology between ‘good and evil’ wasn’t a feature of Israel’s theology until the post-exilic period, particularly with the rise of apocalypticism in the second century BCE. The idea that Satan was an enemy of God who rebelled against God didn’t come about until this period….The serpent in the garden did not come to be seen as ‘the devil’ until the intertestamental period. In ancient mythology, the serpent was a regular fixture in stories about the pursuit of immortality. Just as the serpent in Genesis is responsible for Adam and Eve’s loss of the tree which gave them eternal life, so too in the Epic of Gilgamesh, it is a serpent that steals from Gilgamesh the plant that gives immortality. The serpent was the perfect symbol for this, because a snakebite was lethal.”
Note: For more on this topic, here’s a good blog post by Jacob Wright: http://brazenchurch.com/biblical-satan/