Zoroastrianism is the predominant religion of Iran, and was historically followed by the inhabitants of much of the rest of the Greater Persia region as well. It was founded by the philosopher and prophet Zoroaster, who was born around 1100 BC in the city of Balkh, in modern-day Baktristan.
Zoroastrians believe that the universe is host to a cosmic conflict between two divine forces - Ahura Mazda, from whom all good emanates, and Angra Mainyu who is the source of evil and destruction. The most sacred texts are the Gathas, a collection of hymns thought to be written by Zoroaster himself, and the Avesta, which was composed over several generations. Both are traditionally written in the Avestan language, a continuation of the ancient language spoken by East Iranians in the second millennium BC.
Zoroastrianism was once divided into two main branches - Mazdaism, which was more clearly monotheistic, and Zurvanism, which emphasised the dualistic nature of the religion. Mazdaism was dominant in the western provinces, and for this reason, when Mesopotamia was conquered by the Muslim Romans and Arabs, Mazdaists were initially tolerated as People of the Book but were eventually absorbed into the wider Islamic community.
Because of this apparent collaboration of Mazdaists with the enemies of Persia, the later Sassanid emperors hardened their position against both Islam and Mazdaism, enforced Zurvanism as the sole state religion of Persia, and emphasised Zoroastrianism as a key part of the Persian national identity. It is generally believed that this is the primary reason why Zoroastrianism has survived to this day, and has not faced the same fate as Christianity did in Europe.
Today, most Zoroastrians live in Iran and Baktristan, though scattered remnants are still to be found in Kurdistan, Nejd and the rest of the Iranian world. Zoroastrianism is also followed in many Iranian immigrant communities all over the world.