After Alexander the Great conquered Persia in the late 4th century BCE, Zoroastrianism declined in importance for the next 500 years, apart from the rise of Mithraism, which influenced Christianity. In the 3rd century CE it became the state religion of the Sassanid dynasty, only to succumb to the rise of Islam in the mid-7th century, after which it all but disappeared from the world.
One of the oldest and least understood of the great traditions, Zoroastrianism faces extinction today, with fewer than 150,000 members left. Only small pockets of Zoroastrians remain in Iran, where it was the dominant religion until the coming of Islam. But the tradition is carried on elsewhere by the Parsis (or Parsees, “Persians”), descendants of Iranian refugees who brought Zoroastrianism to India beginning in the 10th century. The Parsis began in India as farmers, but went on to become successful in business and industry. They got along well with the British during the colonial period, and adopted many Western customs, to the extent that some observers feel their practice of the religion is only a dim shadow. North America has a small Zoroastrian community, numbering probably fewer than 10,000. The fact that it has evolved from a credal religion, preached to all who would listen, to an exclusively ethnic faith passed on mainly by birth has not helped its chances of survival.