I form the light, and create darkness. It has been recently denied that there is any allusion in these words, or in those which follow, to the Zoroastrian tenets; and it has even been asserted that the religion of the early Achaemenian kings was free from the taint of dualism.
But according to some authorities, "a god of lies" is mentioned in the Behistun inscription; and the evidence is exceedingly strong that dualism was an essential part of the Zoroastrian religion long before the time of Cyrus.
It is quite reasonable to suppose that Isaiah would be acquainted with the belief of the Persians and Medes, who had come in contact with the Assyrians as early as b.c.. 830; and a warning against the chief error of their religion would be quite in place when he was holding up Cyrus to his countrymen as entitled to their respect and veneration.
The nexus of the words, "I am the Lord, and there is none else. I form the light, and create darkness," is such as naturally to suggest an intended antagonism to the Zoroastrian system. Under that, Ormuzd created "light" and "peace," Ahriman "darkness" and "evil."
The two were eternal adversaries, engaged in an inter-ruinable contest. Ormuzd, it is true, claimed the undivided allegiance of mankind, since he was their maker; bat Ahriman was a great power, terribly formidable—perhaps a god (diva)—certainly the chief of the devas.
It was from Zoroastrianism that Manicheism derived its doctrine of the two principles, and to the same source may, with much probability, be traced the "devil-worshippers" of the Zagros mountain chain.