... It starts with a person named Jarutha described in the Rig Veda in less than complimentary terms, wherein he is against the sage Vasistha. It is also said (RV 7.9.6) that Vasistha had later killed Jarutha, possibly with Agni, fire. In the Rig Veda (7.13.1) Agni is also called the demon (asura) slayer.
In the Vedic texts, Jarutha is also called Jarasabdha. The Bhavishya Purana (Chapters 139-140) describes the history of the Maga Jarasabdha. The word maga refers to a dynasty of priests whom Jarasabdha was a progenitor, born in the family line of vira aditya, the powerful Aditya, meaning the sun god. So worship of the sun was important in their line.
Mandala Seven of the Rig Veda talks about Vasistha’s devotion to Varuna. Varuna was a great god of the rivers and seas, and was also called Asura-maya in the Rig Veda. Asura-maya means lord of the demons, the non-Vedics, because Varuna had power over the demonic undersea creatures. (Asura comes from the Sanskrit asun-rati, meaning he who gives life or rejuvenates, and maya because he measured out the sky. The word asura later became connected with those who were against the Vedic standards.)
Vasistha was fathered by the demigods Varuna and Mitra. It seems that both Jarutha and Vasistha were priests of Varuna, but a rivalry broke out between the two. Because of this rivalry, Jarutha left the Vedic culture and preached something different. He did not accept the Vedic standards and began to promote a different view, which was not accepted by Varuna. Due to this disagreement, Varuna cursed Jarutha and rejected him from Brahminical culture. So Jarutha was expelled from the region of Bharatvarsha, and he went to Persia. There he was able to propagate his own religion and philosophy.
In the region of Persia, Jarutha became known as Zarathustra, and began what became known as Zoroastrianism, called after his other name Zoroaster, which is a Greek derivative from the name Zarathustra. But even historically it is known that his early teachings aroused great hostility towards him.
In ancient Iran, the hereditary priestly caste was called the Magi. So it appears that the lineage of Maga Jarasabdha (Jarutha, Zarathustra) began from Varuna, the chief of the solar deities. In the Zoroastrian Zend Avesta, the name of God is Ahura-mazda, which matches Varuna’s Vedic title of Asura-maya. So, Zarathustra changed what had been the Vedic view into a different philosophy. Ahura Mazda, which means Wise Lord, became the name of the Zoroastrian God.
Furthermore, the Vedic demigods are headed by Brihaspati, whose other name is Angirasa, from where we get the name of Angra Mainyu in Zoroastrianism, who is depicted as the devil or Satan. So the Satan of Zoroastrianism is the Vedic spiritual master of the demigods.
In this way, Jarutha took the side of the demons and, though accepting Varuna as the main god, he promoted the idea that the non-Vedics were superior in their position. Basically, his philosophy was a rejection of the Vedic view, and he refashioned it in his own way. So there were still many Vedic aspects that remained. Even the language in the Zend Avesta is very similar to Sanskrit, and contains much pre-Zoroastrian material that can be related to the Rig Veda.
Nonetheless, Jarutha/Zarathustra called the demons Ahuras, or the pious ones, opposite of the Vedic view of them being called asuras, the demons. He also called them the daivas, similar to the Sanskrit name devas for demigods. In this way, the whole philosophy of Zoroastrianism was to take the Vedic philosophy and turn it upside down. So in Zoroastrianism the power of darkness became the power of light because he agreed with the non-Vedic point of view.
The point is that now we had Angra Mainyu as the name of the devil and Ahura Mazda as the name of God in this new faith. This idea of a God and a counterpart known as the devil, who could threaten the plans of God and divert the good intentions of the pious, now appeared for the first time, and is what filtered down through other religions of that region, including Judaism, Christianity and then Islam. The basis of this whole idea was pure concoction. And this idea and lack of higher spiritual knowledge is what has negatively affected any of the philosophies that followed this premise. This was because Zoroaster had accepted one demigod in contrast to another, rather than accepting the purer form of Lord Vishnu or Krishna as the Supreme, who has no rival. However, even in the Vedic system there may be evil spirits, subtle beings who are misguided and malevolent, and who cause trouble, but they are not beings who can threaten the will of the Supreme Lord."...