Although a figure called satan first appears in the Hebrew Bible, he is a far cry from the devil of medieval Christian (or even Jewish) theology, lore, and iconography, as John Gregory Drummond explains:
In the Hebrew Bible, God’s greatest enemies are not fallen angels commanding armies of demons, or even the gods of other nations, but, rather, human beings. It isn’t the devil that spreads evil across the face of creation—it is mankind. Other than human beings, God has no nemesis, nor are there malevolent spiritual forces not under His authority. God is ultimately a god of justice. He is behind the good and the bad, behind the blessings and the curses. It is within his divine court of justice and retribution that Satan has his origins.
The Hebrew word satan, meaning “accuser” or “adversary,” occurs several times throughout the Hebrew Bible and refers to enemies both human and celestial. When referring to the celestial adversary, the word is typically accompanied by the definite article. He is ha-satan—the Accuser—and it is a job description rather than a proper name. From the Accuser’s appearances in the books of Job and Zechariah, it seems that the job entails calling attention to the unworthiness of mankind. The Accuser is essentially the prosecuting attorney of the divine.
It is difficult to determine at which point in Israel’s history the Accuser began to take on a much more sinister role in the Israelite/Jewish belief structure, or how heaven’s great prosecutor became the “prince of darkness” (in the New Testament, Ephesians 6:12). It is certainly easy to make the connection between Israel’s time in exile and the likely influence of the cosmic dualism of Persian religion. However, even within books written well after the return from foreign lands, the Accuser is still a self-righteous lawyer. . . .
It is perfectly clear, however, that by the 1st century CE, Judaism developed a belief in the divine forces of darkness doing battle against the forces of light. This can be seen within the New Testament and other extra-biblical writings such as those found among the Dead Sea Scrolls.
Read more on Bible History Daily: http://www.biblicalarchaeology.org/daily/biblical-topics/bible-interpretation/who-is-satan/