Zoroastrianism

In the early days, before the emergence of the empire of the Persians and the Medes, the Aryan inhabitants of Iran were polytheists, worshiping, as most other primitive people, natural phenomena such as fire, water, wind, the moon and the sun. However, about seventh century BC, the Zoroastrian religion appeared, descending it appears from   Azarbaijan where Zoroaster himself was born, down to Parsa, then the home of the Achaemenids. The first Achaemenid king to convert to Zoroastrianism was Darius I (550-486 BC). Before him, Cyrus the Great and his son Cambyses, are believed to have followed the religions of their ancestors and the creeds of their subjects. When Zoroastrianism first appeared, it did so as a monotheistic religion with Ahura Mazda as its God, as has been mentioned by Darius in the inscriptions left by him in which he declares all his achievements to have been made possible through the grace of this deity.
The early Zoroastrians buried their dead as evidenced by the tombs of Zoroastrian kings of the Achaemenid dynasty. They had no use for statues to represent their deity; and had no altar for worship, but apparently they sacrificed to God on mountaintops; and they abhorred lies. Avesta is their book of scriptures with Gathas said to be the teachings of Zoroaster himself. Thus, the Persians were one of the first people on earth to adopt monotheism. Some historians in fact tend to believe that Zoroastrianism was a great source of encouragement and confidence in the development of Judaism, because it provided the right background and fostered the notion of monotheism.
Gradually, however, Zoroastrianism became adulterated by other old and new thoughts with Mithras and Anahita appearing as strong deities, though inferior to Ahura Mazda.
When the Greeks, led by Alexander, came to Persia they treated the Persians respectfully because they were impressed by the Persian manners, good taste, etiquette and traditions and under the Seleucids who ruled in Persia after Alexander's death, the Persians were treated as equals to the Greeks and they were allowed to retain their own religion and traditions. Thus Zoroastrianism survived the Seleucids and the Parthians who followed them, though it lost its control as a state religion which it had enjoyed in the Achaemenid era. The Sassanids, who overthrew the Parthians, however, based their statesmanship on the Achaemenid model, developing a strong central government with a firm grip: a strong central government required a strong state religion and thus appeared Mazdaism: Zoroastrianism revived, but somewhat changed. It was no longer a monotheistic religion which it had been at the early stages of its emergence.
Now, Mithras, Anahita and other deities occupied firm places in the new Zoroastrianism which also possessed a powerful clerical hierarchy that dictated the law. Meanwhile, other religions were respected and everyone enjoyed religious freedom, except for Mani's new faith which the Zoroastrian clergy, the Magi, found against their principles and interests and Mani was soon put to death (AD 273) and his followers were badly persecuted. Also, from time to time, depending on the political relations with Byzantium on the west, Persian Christians were persecuted until they broke away from Byzantine Rome and established the Nestorian Christian Church.

Islam
Such were the conditions in Iran when the Muslims were approaching: religious tolerance but a strong clerical hierarchy whose word was the law at the service of the kings and the nobles. There was a rigid class system in which the noble remained so through heredity, and the lower classes were forever denied the chance of social rise no matter how able or clever. And then came the Muslims, with a pure monotheistic faith, which promised equal opportunities to all, the most distinguished men being the most pious, honest and worthy. Here was a new way of life in which a man's status was not based on his birth but on his virtues, which allowed the poor Bedouin Muslim to stand up after the Friday prayer and warn the caliph that if he went astray from "the right path'' the barefooted Bedouin would answer him with his sword.

Shi'ism
However, the Okayed rulers forgot Islamic virtues and resorted to ethnic prejudices and a social hierarchy in which the Arab, not the Muslim, was the master by birth. A worthy Muslim, a great warrior and a brilliant scholar, the Prophet's cousin and son-in-law, Imam (Leader) Ali, reminded the Muslims of true Islamic teachings and stood up to Moawyah who had claimed independence from the caliphate and had established a state in which just the appearance of Islam prevailed. Iranians appreciated Imam Ali, and Shiism was born.
"Shia" means friend or follower, and Shias are friends and followers of Imam Ali and the next eleven Imams who descended from him. The twelfth Imam, Mahdi, disappeared but will return one day to cleanse the world of sin and evil.
Imam Ali's righteousness and sense of justice were too high for his contemporaries to understand and thus he was martyred in a mosque in Kufa while in prayer. His blood "watered the seed of Shiism" that had been freshly sown, and later his son's blood-also martyred-was spilled in the deserts of Karbala, further watering the seed. And in this way developed a branch of Islam which was based on the love of the "ahl-ol-beit" (the people of the Prophet's household) and Imams capable of correctly interpreting the words of God and the Prophet, their grandfather.
Shiism first developed and spread among Iranians but gradually extended all over the Muslim world, but most Muslims have always been Sunnis or "the people of the (Prophet's) tradition" who base their interpretations of the laws on what has been passed on, from one generation to another, about the deeds and words of the Prophet and who claim that since the Prophet had allegedly not given precise instructions regarding his succession, the successor had to be chosen by the community which apparently chose Abu Bakr, a close follower and a staunch supporter of the Prophet from the early days of his rise to prophethood. To these people Abu Bakr is the first successor to the Holy Prophet; to the Shias it is Imam Ali. To the former, the leader of the Muslims, the caliph, is a temporal power; to the second, it is the Imam and holds spiritual powers.
Despite such differences, however, the Sunnis and the Shias agree on the basics of Islam, and remain brothers. Today, by far the largest part of the Iranian population, as well as about half that of both Iraq and Lebanon, plus minorities in Afghanistan, Pakistan, India and East Africa are Shias, followers of Duodecimal (from twelve Imams) Shiism.
As time passed there appeared subdivisions within the Shia sect as a result of disagreements amongst Shias over such matters as the Mahdi being Esmail, the seventh Imam and not the twelfth, which is the reason for dissension by the Ismailis or Ismailites. In fact there are quite a number of "isms" within he Shia sect, but the central idea, the righteousness of "the people of the house" and their power for interpretation of the words of God and the Holy Prophet, is common to all sub-sects for whom the Prophet; Fatima, his beloved daughter; Imam Ali, his cousin and son-in-law; Imam Hassan, Ali's older son and Imam Hussein the younger brother, remain as the "five (holy) persons", revered and adored. Furthermore, to Shias learning from their Imams, martyrdom for justice and truth is a basic principle.

Islam  sunni

Sunni Muslims are the largest denomination of Islam and are known as Ahl as-Sunnah wa’l-Jamā‘h or simply as Ahl as-Sunnah. The word Sunni comes from the word sunnah, which means the teachings and actions or examples of the Islamic prophet, Muhammad. Therefore, the term "Sunni" refers to those who follow or maintain the sunnah of Muhammad. In many countries, overwhelming majorities of Muslims are Sunnis, so that they simply refer to themselves as "Muslims" and do not use the Sunni label.

The Sunnis believe that Muhammad did not specifically appoint a successor to lead the Muslim ummah (community) before his death, and after an initial period of confusion, a group of his most prominent companions gathered and elected Abu Bakr Siddique, Muhammad's close friend and a father-in-law, as the first caliph of Islam. Sunni Muslims regard the first four caliphs (Abu Bakr, `Umar ibn al-Khattāb, Uthman Ibn Affan and Ali ibn Abu Talib) as "al-Khulafā’ur-Rāshidūn" or "The Rightly Guided Caliphs." Sunnis also believe that the position of caliph may be attained democratically, on gaining a majority of the votes, but after the Rashidun, the position turned into a hereditary dynastic rule because of the divisions started by the Umayyads and others. After the fall of the Ottoman Empire in 1923, there has never been another caliph as widely recognized in the Muslim world. 

Sufi Mysticism
Sufi Mysticism or "tassawof" is a discovery made by Iranians within Islam, derived from the Quranic verses. It concentrates on the highest spiritual aspects of Islam. According to Sufis, it is foolish to try to prove the existence of God through logic and reasoning only; God must be felt; He is a light that must shine in the believer's heart and the heart must be pure enough to receive the light, in which case the two can become one; indeed the two are the same, but separated: Man's soul is on exile from the Creator and is in anguish, longing to return "home" to lose himself again in Him. Here lies the crux of Sufism or Sufi Mysticism.
But to join the Creator, one has to be worthy; it needs conscious effort and strife. First, one must realize the transience the insignificance of material life and follow the path leading him towards higher and higher levels of spiritual life until one is pure enough to reach "home" and join Him again. But the path is a difficult one; the soul, while in mortal flesh, must suffer and yearn to be back.
Some of Iran's greatest thinkers, poets and scholars have had more or less Sufi Mystic tendencies and the greatest of them are Sohrevardi, Ghazali, Mowlavi, Hallaj, Hafez and Saadi. Hafez, in one of his poems, likens man to a falcon which flies away from his "home" to the city of miseries and is called back all the time and he wonders why the falcon hesitates to return. Saadi says that his "Friend" is nearer to him than himself, yet is separated from him.
Unfortunately Sufism has been contaminated with stories about ridiculous miracles said to have been performed by the great Sufis, never claimed by them. It has also been used as an escape for negligence of Islamic rites and rituals. his is not the true nature of Sufism which seeks nothing but God's satisfaction, and does not free a man from his duties as a Muslim.

Christianity and Judaism
Throughout history Iranians have shown great tolerance towards other peoples' religious beliefs, and creeds; and since the adoption of Islam they have been particularly tolerant of Christians and Jews, who are "peoples of the book". Christian, Jewish and Zoroastrian scholars and scientists worked side by side their Muslim counterparts, as did for example Al-Masihi with Abu Ali Sina (Avicenna).
Today, Armenian, Assyrian and other Christians, as well as Jews and Zoroastrians enjoy complete religious freedom in Iran although they are expected to observe Islamic codes of public conduct. They are also represented at the Majlis (parliament). Furthermore, many members of these religions fought side-by-side Muslim Iranians in the Constitutional Uprising of the late 19th century, which finally resulted in the Constitution of 1906.