The very first monotheistic religion was Zoroastrianism which was founded sometime around 1500 BC in what is now Iran.
“Zoroastrianism is the oldest of the revealed world-religions, and it has probably had more influence on mankind, directly and indirectly, than any other single faith.”
“Zoroaster was thus the first to teach the doctrines of an individual judgment, Heaven and Hell, the future resurrection of the body, the general Last Judgment, and life everlasting for the reunited soul and body. These doctrines were to become familiar articles of faith to much of mankind, through borrowings by Judaism, Christianity and Islam.”
Mary Boyce, Zoroastrians: Their Religious Beliefs and Practices (London: Routledge and Kegan Paul, 1979)
Zoroastrianism was the religion of the Persian Empire, the reigning power in the Middle East during early biblical times. It was also the first religion that embodied a moral code that focused on dualisms of right and wrong, good and evil, light and dark as they played out both in the heavens and in the hearts of human beings. Thus it was the first religion that did not simply focus on the rituals surrounding death.
The original Gathas are said to have been written by Zoroaster himself. (He is also called Zarathustra.) Zoroaster’s teachings focused on personal responsibility and assumed that people had been given the gift of free will so that they could make their own decisions. They defined the one God, Ahura Mazda as embodying all good qualities while evil was defined simply as the misuse of free will.
Thus, evil in the original version of Zoroastrianism was a human quality and not a god in and of itself. This leaves the One God in charge of both aspects of the moral universe. There was originally no god of evil (devil) to act in opposition to Ahura Mazda. This ancient distinction is the key to understanding the true nature of the spiritual realm.
Human beings live in a material world, and they are naturally inclined to see evil as a material substance that can cling to people, places and things rather than the spiritual quality embodied by the pure religious philosophy of Zoroaster. Thus, shortly after the death of Zoroaster, the younger generations introduced a god of evil (Angra Mainyu) who opposed Ahura Mazda.
This was a very important event because the introduction of a god of evil is a recurring, and very natural theme throughout the history of every religion and every civilization. It is difficult for ordinary people who live in material bodies to conceive of a purely spiritual world without adding familiar material trappings to it, and the addition of a god of evil allows them to do this.
As civilizations evolve, there is always a tendency to modify the original idyllic religious forms by inserting more and more materialistic philosophy. Evil stops being a human quality that exists because the human has turned away from God and is transmuted into a form of matter personified by the Devil.