Zoroastrianism is a religion started by Zoroaster in 6th century BC
Ancient Persians followed Zoroastrianism. After the advent of Islam they converted to Islam. Those who did not embrace Islam fled to India where that religion is still practiced and they are called Parsis. Zen Avesta is their Holy book.
Zoroastrianism differed from previous religions in that it introduced a concrete morality. It is thought that this was the first known religion to speak of good and evil and the struggle between the two.