The devil makes his debut (biblically speaking) in the Old Testament where he works as one of god's agents. Called 'Satan' or hassatan his name literally translates to 'the adversary' and his godly duties are to roam the earth searching for the wicked and tempting the pious. BUT he and god are pals... They do fun things together like torment Job and kill his family on a bet. The evil version comes in the next book...
The New Testament switches things up a bit. Christ (or the gospels) use the devil as a political agent to 'demonize' those who stood in their way. The Gospels are notoriously anti-Semitic in their accusations of devilry from the Pharisees and Sadducees. But they are oddly kind to Rome. Makes sense: why pick on the Romans when they have nothing to lose and everything to gain by demonizing the Jews? Jesus is tempted by the devil with all of earth's spoils (all of this I give to you...), turns down the offer (for whatever god-forsaken-reason) and goes on to be nailed to a cross and die. The devil is officially a bad guy -- he is the opposing force working against mankind to tempt us into sin and depravity. By the time we get to Revelation (a book originally thought to be written by John the Baptist but likely written by an exiled monk in Greece dubbed John of Patmos) we are faced with a terrifying dragon who rapes virgins. It's a confusing book and really seems to come out of nowhere, but it's a lot of fun to read compared to the REST of the bible.
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