The Satan of the Book of Job is not the Devil. He is ha-satan — the Hebrew definite article ha (the) combined with satan (adversary, accuser, prosecutor) — a title, not a proper name. He is a member of the divine court (the bene ha-elohim, the sons of God) who appears before God with a specific function: he is the prosecuting attorney of the heavenly assembly, the one whose job is to challenge, to test, to investigate whether human virtue is genuine. When God praises Job's righteousness, ha-satan raises the obvious legal objection: "Does Job fear God for nothing? Have you not put a hedge around him and his household and everything he has?" It's a fair question. The figure who becomes the supreme enemy of humanity began as a divine functionary with a legitimate professional role.
The word satan (שָׂטָן) in Hebrew means adversary, accuser or one who opposes. It is used in the Hebrew Bible in several non-supernatural contexts: in Numbers 22, an angel of the LORD stands as a satan (an obstacle) in the path of Balaam. In 1 Samuel 29, Achish worries that David might become a satan (an adversary) against the Philistines in battle. In 1 Kings 11, God raises up several satans against Solomon — human political enemies. The word describes a function, not an ontology.
When the Hebrew Bible uses ha-satan with the definite article (as in Job and Zechariah 3), it refers to a specific figure with a specific court function. This figure is not God's enemy — he is God's employee. He does not act without God's permission: in Job, God specifically authorises each stage of Job's testing. The tension in Job is not between God and Satan as opposing forces but between God's sovereignty and the question of whether human virtue is real or merely prudential.
The transformation into the Devil: the development of Satan from divine prosecutor to cosmic enemy took place gradually across the Second Temple period (516 BCE – 70 CE), influenced significantly by the dualistic theology of Zoroastrianism that Jewish communities encountered during the Babylonian exile and Persian period. In texts of the Second Temple period — particularly 1 Enoch, Jubilees and the Dead Sea Scrolls — Satan (and his equivalents Mastema, Belial, Azazel) develops into an active opponent of God and humanity rather than a court official with a prosecutorial function. By the time of the New Testament, Satan is the "prince of this world" (John 12:31), the "ruler of the kingdom of the air" (Ephesians 2:2), the adversary who "prowls around like a roaring lion looking for someone to devour" (1 Peter 5:8). The evolution from functionary to enemy was complete.
Islamic tradition has its own complex relationship with the adversary figure. Iblis (إِبْلِيس) — from the Arabic root meaning "to despair" or possibly from the Greek diabolos — is the figure who refused God's command to bow before Adam. In the Quranic account, Iblis argues that he is made of fire while Adam is made of clay — fire is superior to clay, and therefore the command to bow is unjust. This is a logical rather than malicious objection, and the Quran's account of Iblis has generated significant philosophical discussion about whether his refusal was the ultimate pride (placing his own judgment above God's command) or the ultimate fidelity to monotheism (refusing to bow before anything but God).
Iblis is given respite until the Day of Judgment, during which he works to lead astray those humans who prove susceptible to temptation — with the explicit statement that he has no power over those who are sincere believers. Shaitan (شَيْطَان, from the same root as the Hebrew satan) is used in the Quran more broadly to describe the whispering tempter, and the Surah Al-Nas (114) — the final chapter of the Quran — is an invocation for protection from "the evil of the retreating whisperer who whispers in the breasts of mankind, from among jinn and people."