Originally a Pagan god, taken by the Christians during the Crusades and Secular State ages and turned into Christianity's adversary.
Happened right around the time the Bible was picked and prodded together, rejecting earlier first hand testaments like that of Judas and Mary Magdalene.
Church: "Our numbers are downeth, ask thee marketing teameth what thee other religions, are doing that we aren'teth doing"
Servant: "It appears thine haveth entities that representeth good AND evil. One god by the nameth of Satan, is used to represent some sort of wrongeth doing to others or some sorteth."
Church: "Thine haveth a god for evil? Well, we'll just tell everyone he's OUR god's enemy, and worshipping thee gets you in someth sort of celestial jail."
Servant: "Celestial jail, sir?"
Church: "What could we calleth it?"
Servant: "Hell, I don't knoweth."