Why is there a God and a devil?

Answer 1

Some scholars have pointed out that the concept of Satan did not enter Judaism until after the Babylonian Exile, when the followers of Judaism became exposed to the ideas of Zoroastrianism and returned with some similar beliefs. Strangely, Satan entered Judaism as a loyal servant of God, not as his adversary. In Christianity Satan is the devil, the adversary of God, much as in Zoroastrianism, where Angra Mainyu is the adversary of Ahura Mazda; equally uncreated, but ignorant and wholly malign. There is a God and a devil, through the process of syncretism of ancient faiths.