The idea of "Satan" as the "strong adversary of God" seems to go back to Zoroastrianism / Manicheanism. -- RJwiki

Yes it does. And Augustine was definitely a Manichean originally. He changed his mind. But it influenced him a lot.
Also there's all the "evil Gods" in all those pagan religions that the Catholics absorbed (renaming the gods saints or devils based on whatever worked at the time). I guess there were some they didn't want to say were agents of the main God...

Satan is not a god

The idea that Satan is another god is a misconception. According to Christian theology, Satan or Lucifer was an angel who defied God and was cast out of heaven. He was a servant, created by God, who rebelled against Him, not a being of equal or similar power to God. -- Augustgrahl 02:16, 13 December 2006 (UTC)