The Jewish, Christian, and Muslim religions are monotheistic faiths, which means their followers believe there is only one God. That God has a powerful adversaryenemy; opponent known as Satan, or the Devil. Satan's role changed over time, as the three religions developed. At first he was a creature under God's control with the task of testing people's faith. In time, however, Satan came to be seen as the prince of darkness, ruler of all evil spirits, enemy of both God and humankind, and source of treachery and wickedness.

From Adversary to Devil. The name Satan comes from a Hebrew word meaning "adversary." It first appears in the Hebrew Bible, or Old Testament. In the book of Job, God allows this adversary—sometimes called Samael in Jewish literature—to heap misfortunes on Job to see whether Job will turn against God. Judaism was influenced by...