Excerpts from Books and Wikipedia
"Named after the Persian prophet Zoroaster (also known as Zarathustra), Zoroastrianism is arguably the world’s first monotheistic religion. It’s still practiced today as a minority religion in parts of Iran and India. Zoroaster, who likely lived sometime between 1500 and 500 B.C., taught followers to worship one god instead of the many deities worshipped by earlier Indo-Iranian groups. The Achaemenian kings were devout Zoroastrians. By most accounts, Cyrus the Great was a tolerant ruler who allowed his subjects to speak their own languages and practice their own religions. While he ruled by the Zoroastrian law of asha (truth and righteousness), he didn’t impose Zoroastrianism on the people of Persia’s conquered territories. Hebrew scriptures praise Cyrus the Great for freeing the Jewish people of Babylon from captivity and allowing them to return to Jerusalem. Subsequent rulers in the Achaemenid Empire followed Cyrus the Great’s hands-off approach to social and religious affairs, allowing Persia’s diverse citizenry to continue practicing their own ways of life. This period of time is sometimes called the Pax Persica, or Persian Peace." [History.com Editors]