The state religion of Iran is Ja'fari Faith of the Twelve Imams. According to official 1986 statistics, about 98.5% of the population is Muslim, most of whom belong to Shi'ite Sect. Other Islamic sects are Hanafi, Shafe'i, Maleki, Hanbali, and Zeidi, all of which are respected and enjoy perfect freedom. The Zoroastrian, Armenian, Jewish, Assyrian, and Chaldean religious minorities have their own religious organizations.
Islam
The Arabic word for "submission" to the will of the God (Allah), Islam is the name of the religion, which originated in Arabia during the 7th century AD through the Prophet Muhammad. Followers of Islam are known as Muslims, and their religion embraces every aspect of life. They believe that individuals, societies and governments should all be obedient to the will of God as it set forth in the Koran, which they regard as the word of God revealed to his Messenger, Muhammad. The Koran teaches that God is one and has no partners. He is the Creator of all things, and holds absolute power over them. All persons should commit themselves to a life of grateful and praise of Resurrection they will be judged. Those who have obeyed God's commandments will dwell forever in Paradise, but those who have sinned against God and not repented will be condemned eternally to the fires of Hell.
Since the beginning of creation God has sent prophets, including Moses and Jesus, to provide the guidance necessary for the attainment of eternal reward, a succession culminating in the revelation to Muhammad of the perfect word of God.
There are five essential religious duties known as the "Pillars of Islam": (1) The Shahada (profession of faith) is the sincere recitation of the two-fold creed: "There is no God but God" and "Muhammad is the Messenger of God". (2) The Salat (formal prayer) must be performed at fixed hours five times a day while facing towards the Holy City of Mecca. (3) Alms-giving through the payment of Zakat ('purification') is regarded primarily as an act of worship, and is the duty of sharing one's wealth out of gratitude of God's favor, according to the uses laid down in the Koran. (4) There is a duty to fast (Saum) during the month of Ramadan. (5) The Hajj or pilgrimage to Mecca is to be performed if at all possible at least once during one's lifetime. Shari is a sacred law of Islam, and applies to all aspects of life, not just religious practices. It describes the Islam way of life, and prescribes the way for a Muslim to fulfill the commands of God and reach Heaven. There is an annual cycle of festivals, including Hijra, the beginning of Islamic year, and Ramadan, the month during which Muslims fast during the hours of daylight.
There are two basic groups within Islam. Sunni Muslims are in the majority, and they recognize the first four caliphs as Mohammed's legitimate successors. The Shiites comprise the largest minority group, and regard the imam as the principal religious authority. There are a number of subsects, including the Ismailis (one group of which, the Nizaris, regard the Agha Khan as their imam), and the Wahhabis, an apparently reform movement begun in the 18th century. There are over one billion Muslims throughout the world.
Religious Minorities
While Muslims constitute more than 98.5% of the country's population, the Christians command a population of 0.7%, the Jews 0.3%, the Zoroastrians 0.1%, and the followers of other religions 0.1%. Protection of the cultural, social, and political rights of all religious minorities are recognized by the Constitution of the Islamic Republic of Iran.
Iranian Jews, apart from being represented in the Parliament, have around 30 synagogues all over the country, their own association, a journal named Tamous, and private schools.
The Armenians and Assyrians, who have peacefully lived in Iran since ancient times, constitute Iran's Christian community. Armenians, like other minorities, have their own private schools, a journal in Armenian named Alik, and are represented in the Parliament. There are around twenty Gregorian Churches where religious ceremonies take place, as well as a Catholic Church, and other centers for Protestant Armenians.
The Assyrians have their own association, which takes care of their community, churches where religious ceremonies take place, and a number of publication centers. They are also represented in the Parliament.
There are other sects of Christianity like Protestants, Roman Catholics, Adventists, all of whom have their own centers of worship.
Zoroastrianism prevailed in Iran much earlier than the advent of Islam. Zoroastrianism is a dualistic ancient Iranian religion the creation of which is credited to the mythical prophet. Zarathushtra (Zoroaster in Greek). Zoroastrianism had been fully shaped by the 7th century BC. The main thing in Zoroastrianism is the doctrine of the constant struggle in the world between two opposite elements: good, personified by the god of lightness Ahura Mazda (Ormanzd), and evil, personified by the god of darkness Angro Mainyush (Ahriman). Its eschatological ideas on the end of the world, judgment, resurrection of the dead and a future savior born of a Virgin exerted great influence on Judaism and Christianity. Apart from Iran, at present, exists in the form of Parsiism in India and has preserved the old dualistic ideas but developed the concept of a single Almighty God.
References : Salam Iran Website