Who is Satan? What does Satan do? According to popular imagination and centuries of Christian theology, Satan is the archenemy of God, the opponent of all that is good; Satan does evil. Bible scholars, to their credit, now recognize that the biblical authors conceived of the nature and activity of the figure somewhat differently from later theologians. According to the present scholarly consensus, the early literature portrays Satan or the satan as he is referred to in the Hebrew Scriptures) as “the Adversary,” or, more specifically, “the Accuser.” He serves God. This God vs the Devil – is a theological concept obtained by the Abrahamic religions from Zoroastrianism.
Note: Satan is also synonymous (in one way or another) with fallen angel, devil, demon, and many other terms referencing ‘darkness’ and an enemy of God. But long, long ago ‘satan’ (devil) was not understood in that manner. It was not a ‘person’ or a ‘personification’ but a Title and a Function or the natural order of ‘the world’, i.e. ‘The Satan’.
Every reference in the Old Testament to the one called “Satan [שָׂטָן]” is actually “hassatan [הַשָּׂטָן].” The “ha” in Hebrew is the definite article “the.” This means that, when hassatan is used, it is a title or office, not a proper name: the satan not Satan. In Hebrew as in English, proper names do not have the article “the” attached to them. Hassatan Means “the Adversary,” Satan is an adversary who tests humanity’s faith in God-References to Hassatan in the Hebrew Bible-In the Book of Job, Satan is referred to as Hassatan and works for God by testing humanity, Satan is seen as the Adversary or Prosecutor, an agent of God, that searches out the individual’s wrongdoings and appears as their accuser.
Listen/watch Dr. Christopher Rollston give some background:
The idea of Satan an individual evolved during the height of the Persian Achaemenid Empire (beginning c. 550 BCE) and was adopted by Jews living under Persian rule at the time. His formal name, Satan, derives from the Hebrew ‘ha-Satan’. ‘Ha’ means ‘the’ and ‘Satan’ means ‘opposer’ or ‘adversary’. The name described his eventual function as the opposer of God’s creation. Greek ‘diabolos’, English ‘devil’, meant ‘accuser’, ‘slanderer’, again describing his role. The concept of Satan emerged over time and in phases.
A figure known as ha-satan (“the satan”) first appears in the Hebrew Bible as a heavenly prosecutor, subordinate to Yahweh (God), who prosecutes the nation of Judah in the heavenly court and tests the loyalty of Yahweh’s followers. During the intertestamental period, possibly due to influence from the Zoroastrian figure of Angra Mainyu, the satan developed into a malevolent entity with abhorrent qualities in dualistic opposition to God. In the apocryphal Book of Jubilees, Yahweh grants the satan (referred to as Mastema) authority over a group of fallen angels, or their offspring, to tempt humans to sin and punish them. (Wikpedia).
The Hebrews, Satan, and the Invention of “Lucifer”
The Hebrew scriptures, and Jewish theology, views Satan radically differently than does much Christian theology.
1) The character Satan or “Ha Satan” translates literally as “the accuser” or, in other contexts, “adversary”, and that is what he is: he either tempts humans or brings their sins to God’s attention
2) Satan is not the force of evil; God is the source of all things, including what humans conceive of as good and evil
3) Satan works for, not against, God
4) There are no references to Satan in the earlier books of the Hebrew Scriptures.
5) There are very, very few references to Satan in the later books, and all of these are in just four of the later books: Chronicles, Job, Psalms, and Zechariah, written c. 600-500 BCE. Satan only occurs as a specific character once, however: in the book of Job.
Even today most Jews find the Medieval conception of Satan being the force of evil a rather blasphemous idea because it implies that God is not omnipotent; that is, it implies that God is not the force behind all things, both good and evil. See here for more on Jewish conceptions of The Satan.
“Lucifer” as Satan
One of the reasons contemporary Christians believe there are references to Satan in the older books of the Hebrew Scriptures is due to a misunderstanding of the word “Lucifer”. It’s worth covering this in depth because it shows how radically translation can change our perception of meaning.
Isaiah 14:12: “Lucifer” and “Morning Star” (Venus)
The Hebrew Scriptures (Tanakh) reads like (translates as) this:
“How are you fallen from heaven,
O Shining One, son of Dawn!
How are you felled to earth,
O vanquisher of nations!”
[and the footnote often reads “A character in some lost myth.”]
The pre-Christian Septuagint Greek version of Isaiah 14:12 uses the phrase “ho heosphoros,” which translates as “morning star” (the star we call Venus). This is similar to another Greek name for this star “phosphorus”, which means “burning bright” (Venus is the third brightest object in the sky).
The latin name for this star is “Lucifer” and has the same root as lux; it essentially simply means “burning bright” or “bright light” or “day star”…so when the Greek gets translated into Latin….
Martin Luther’s German version (c. 1534) had “schoener Morgenstern,” that is, “beautiful morning star” as the translation of the Hebrew phrase heylel ben-shachar.
The King James (1611) reads like this:
“How you are fallen from heaven,
O Lucifer, son of the morning!
How you are cut down to the ground,
You who weakened the nations!”
[In the original margin notes of the KJV, however, the original translators included the note that it could also read “O day-starre”.]
Because the KJV was used for hundreds of years as the basic English Bible, most modern versions continued to use the word “Lucifer”, and they kept the Old English style of capitalizing it as a formal name. In the 1600s this word began to be used as a synonym for “Satan”.
And that is how it has passed into not only our own language but our very conception of not only evil but of how the Jews thought of Satan. But Jews do not conceive of Satan as the force of evil, much less as “Venus”.
However, Jesus was also referred to as the “Morning Star”:
“I, Jesus, have sent my angel to give you  this testimony for the churches. I am the Root and the Offspring of David, and the bright Morning Star.”
2 Peter 1:19,:
And we have the word of the prophets made more certain, and you will do well to pay attention to it, as to a light shining in a dark place, until the day dawns and the morning star rises in your hearts.
NIV: Revelation 2:28:
I will also give him the morning star.
History of the Devil
by Paul Carus, 
THE PHILOSOPHICAL PROBLEM OF GOOD AND EVIL.
Mythology being always a popular metaphysics, it is a matter of course that the idea of evil has been personified among all nations. There is no religion in the world but has its demons or evil monsters who represent pain, misery, and destruction.
In Egypt the powers of darkness were feared and worshipped under various names as Set or Seth, Bess, Typhon, etc. Though the ancient Gods of Brahmanism are not fully differentiated into evil and good deities, we have yet the victory of Mahâmâya, the great goddess, over Mahisha, the king of the giants. Buddhists call the personification of evil Mâra, the tempter, the father of lust and sin, and the bringer of death. Chaldean sages personify the chaos that was in the beginning, in Tiamat, he monster of the deep. The Persians call him Angra Mainyu or Ahriman, the demon of darkness and of mischief, the Jews call him Satan the fiend, the early Christians, Devil (διάβολος), i. e., slanderer, because, as in the story of Job, he accuses man, and his accusations are false. The old Teutons and Norsemen called him Loki. The Middle Ages are full of devils, and demonologies of the Japanese and Chinese are perhaps more extensive than our own.
The evolution of the idea of evil as a personification is one of the most fascinating chapters in history, and the changes which characterise the successive phases are instructive. While the old Pagan views survive in both Hebrew and Christian demonologies, we are constantly confronted with accretions and new interpretations. Franz Xaver Kraus, in his History of Christian Art concedes that our present conception of the demons of evil is radically different from that of the early Christians. He says:
“The popular conceptions of the early Christians concerning devils are essentially different from those of the present time. The serpent or the dragon as a picture of the Devil appears not only in the Old Testament (Genesis iii. 1), but also in Babylonian literature, in the Revelation of St. John (xii. 9), and in the Acts of the Martyrs. We read in the Vision of Perpetua: “Under the scales themselves [i. e., for weighing the souls] the dragon lies, of wonderful magnitude.'”
The intellectual life of mankind develops by gradual growth. The old views are, as a rule, preserved but transformed. There is nowhere an absolutely new start. Either the main idea is preserved and details are changed, or vice versa, the main idea is objected to while the details remain the same.
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How the Serpent in the Garden Became Satan
Adam, Eve and the serpent in the Garden of Eden
Introduced as “the most clever of all of the beasts of the field that YHWH God had made,” the serpent in the Garden of Eden is portrayed as just that: a serpent. Satan does not make an appearance in Genesis 2–3, for the simple reason that when the story was written, the concept of the devil had not yet been invented. Explaining the serpent in the Garden of Eden as Satan would have been as foreign a concept to the ancient authors of the text as referring to Ezekiel’s vision as a UFO (but Google “Ezekiel’s vision” now, and you’ll see that plenty of people today have made that connection!). In fact, while the word satan appears elsewhere in the Hebrew Bible/Old Testament, it is never a proper name; since there is no devil in ancient Israel’s worldview, there can’t yet have been a proper name for such a creature.
The noun satan, Hebrew for “adversary” or “accuser,” occurs nine times in the Hebrew Bible: five times to describe a human military, political or legal opponent, and four times with reference to a divine being. In Numbers 22, the prophet Balaam, hired to curse the Israelites, is stopped by a messenger from Israel’s God YHWH, described as “the satan” acting on God’s behalf. In Job, “the satan” is a member of God’s heavenly council—one of the divine beings, whose role in Job’s story is to be an “accuser,” a status acquired by people in ancient Israel and Mesopotamia for the purposes of particular legal proceedings. In Job’s case, what’s on trial is God’s assertion that Job is completely “blameless and upright” vs. the satan’s contention that Job only behaves himself because God has rewarded him. God argues that Job is rewarded because he is good, and not good because he is rewarded. The satan challenges God to a wager that if everything is taken away from poor Job, he won’t be so good anymore, and God accepts. Though a perception of “the satan” as Satan would make this portrait of God easier to swallow, the story demonstrates otherwise; like Yahweh’s messenger in Numbers 22, this satan acts on YHWH’s instructions (and as a result of God’s braggadocio) and is not an independent force of evil.
In Zechariah 3, the prophet describes a vision of the high priest Joshua standing in a similar divine council, also functioning as a tribunal. Before him stand YHWH’s messenger and the satan, who is there to accuse him. This vision is Zechariah’s way of pronouncing YHWH’s approval of Joshua’s appointment to the high priesthood in the face of adversarial community members, represented by the satan. The messenger rebukes the satan and orders that Joshua’s dirty clothing be replaced, as he promises Joshua continuing access to the divine council. Once again, the satan is not Satan who we read about in the New Testament.
The word satan appears only once without “the” in front of it in the entire Hebrew Bible: in 1 Chronicles 21:1. Is it possible that we finally have Satan here portrayed? 1 Chronicles 21 parallels the story of David’s census in 2 Samuel 24, in which God orders David to “go number Israel and Judah” and then punishes king and kingdom for doing so. The Chronicler changes this story, as he does others, to portray the relationship between God and David as uncompromised; he writes that “a satan stood up against Israel and he provoked David to number Israel” (1 Chronicles 21:6–7; 27:24). Although it is possible to read “Satan” here instead of “a satan” (Hebrew uses neither uppercase letters, nor indefinite articles, e.g., “a”), nothing else in this story or in any texts for another 300 years indicates that the idea of an evil prince of darkness exists in the consciousness of the Israelites.