Many believe that the book of Job is the oldest Biblical book. In it, Satan is not the same as most think of today.

The Satan of the Biblical book "Job":

Job's Satan
In the Book of Job, ha-Satan is a member of the divine council, "the sons of God" who are subservient to God. Ha-Satan, in this capacity, is many times translated as "the prosecutor", and is charged by God to tempt humans and to report back to God all who go against His decrees. At the beginning of the book, Job is a good person "who feared God and turned away from evil" (Job 1:1), and has therefore been rewarded by God. When the divine council meets, God informs ha-Satan about Job's blameless, morally upright character. Between Job 1:9–10 and 2:4–5, ha-Satan merely points out that God has given Job everything that a man could want, so of course Job would be loyal to God; if all Job has been given, even his health, were to be taken away from him, however, his faith would collapse. God therefore grants ha-Satan the chance to test Job.[11] Due to this, it has been interpreted that ha-Satan is under God's control and cannot act without God's permission. This is further shown in the epilogue of Job in which God is speaking to Job, ha-Satan is absent from these dialogues. "For Job, for [Job's] friends, and for the narrator, it is ultimately Yahweh himself who is responsible for Job's suffering; as Yahweh says to the 'satan', 'You have incited me against him, to destroy him for no reason.'" (Job 2:3) [7]