Zoroastrianism, the most important religion of ancient Iran, was preached by the prophet Zarathustra (called Zoroaster by the Greeks) who lived probably in the 10th century BC. His religion was adopted by the Achaemenid king Cyrus the Great around 550 BC and it continued to grow under the Achaemenid, Parthian and Sassanian dynasties. Only with the Islamic conquest of Iran did its influence wane.
Zoroastrian priests, known as 'magi', directly served the king and his government. They conducted many rituals with fire, which was a symbol of righteousness; Zoroastrians had a duty to pray before a fire altar five times a day.
The religion also has strength in its doctrine and theology. Its supreme deity, Ahura Mazda, who was all-wise and good, created the world. The evil one, Angra Mainyu, entered the world, bringing evil and destruction, and making it a battleground between good and evil.
It was the collective responsibility of humanity to drive out Angra Mainyu and his attendant evils, through the personal practice of good thoughts, words and deeds. By these means individuals could also attain paradise with the good god; but a fiery hell awaited evildoers.
Although Zoroastrians had much in common with Christianity, Christians and heretical Manichaeans were intermittently persecuted by them during the Sassanian period.
