
Though initially beginning as a political movement, infallibility and sinlessness of the imams later evolved as a distinct belief of (non-Zaidi) Shiism. Twelver and Ismaili Shia Muslims also attribute the quality to Imams as well as to Fatimah, daughter of Muhammad, in contrast to the Zaidi, who do not attribute 'ismah to the Imams. Muslims believe that Muhammad and other prophets in Islam possessed ismah. Ismah is the concept of infallibility or "divinely bestowed freedom from error and sin" in Islam. A more detailed discussion of this verse is available. The basis of Ali as the "wali" is taken from a specific verse of the quran The three clauses of the Shia Shahada thus address tawhid (the unity of God), nubuwwah (the prophethood of Muhammad), and imamah (imamate, the leadership of the faith).

This phrase embodies the Shia emphasis on the inheritance of authority through Muhammad's lineage. The Sunni Shahada states There is no god except God, Muhammad is the messenger of God, but to this the Shia append Ali is the Wali (custodian) of God, علي ولي الله. The Shia version of the Shahada, the Islamic profession of faith, differs from that of the Sunni. The 4th century AH /10th century CE has been referred to by Louis Massignon as "the Shiite Ismaili century in the history of Islam". The first Shia governments and societies were established by the end of the 3rd century AH/9th century CE. The original Shia identity referred to the followers of Imam Ali, and Shia theology was formulated in the 2nd century AH, or after Hijra (8th century CE). Shia Islam embodies a completely independent system of religious interpretation and political authority in the Muslim world. The Shia Islamic faith is vast and inclusive of many different groups. Shia scholars emphasize that the notion of authority is linked to the family of the prophets as the verses 3:33,34 show: "Indeed, God chose Adam and Noah and the family of Abraham and the family of 'Imran over the worlds – (33) Descendants, some of them from others. For the Shia, this conviction is implicit in the Quran and the history of Islam. Al-Shahrastani expresses that the term Shia refers to those who believe that Ali is designated as the Heir, Imam and caliph by Muhammad and that Ali's authority is maintained through his descendants. Nawbakhti states that the term Shia refers to a group of Muslims who at the time of Muhammad and after him regarded Ali as the Imam and Caliph. At present, the word refers to the Muslims who believe that the leadership of the community after Muhammad belongs to Ali and his successors. The term was first used during Muhammad's life. Shiʻa, Shia, Shiʻism/Shiʻite or Shiism/Shiite are the forms used in English, for adherents, mosques, and things associated with the religion. Ĭollectively, adherents of Shia Islam are called the Shīʿah ( Arabic: شِيعَة ), which is short for Shīʿatu ʿAlī ( Arabic: شِيعَة عَلِيّ /ˈʃiːʕatu ˈʕaliː/) meaning "followers of Ali", "faction of Ali", or "partisans of Ali" Shīʿī ( شِيعِيّ) denotes both the singular noun and the adjective form, while Shīyāʿ ( شِيَاع) refers to the plural noun. Twelver Shia is the largest branch of Shia Islam, comprising about 85% of all Shia Muslims. Shia Islam is the second largest branch of Islam, followed by 10–15% of all Muslims. Although there are many Shia subsects, modern Shia Islam has been divided into two main groupings: Twelvers and Ismailis, with Twelver Shia being the largest and most influential group among Shia. This belief later developed into the concept of Imamah, the idea that certain descendents of Muhammad, the Ahl al-Bayt, are rightful rulers or Imams, whom Shia believe possess special spiritual and political authority over the Muslim community. Shia believe Ali should have been the successor to Muhammad as Islam's spiritual and political leader.

Shia Islam is based on a hadith concerning Muhammad's pronouncement at Ghadir Khumm. Adherents of Shia Islam are called Shia Muslims, Shi'ites, or simply Shia or Shi'a. This view primarily contrasts with that of Sunni Islam, whose adherents believe that Muhammad did not appoint a successor before his death and consider Abu Bakr, who was appointed caliph by a group of senior Muslims at Saqifah, to be the first rightful caliph after Muhammad. It holds that the Islamic prophet Muhammad designated Ali ibn Abi Talib as his successor and the Imam (spiritual and political leader) after him, most notably at the event of Ghadir Khumm, but was prevented from succeeding Muhammad as the leader of the Muslims as a result of the choice made by some of Muhammad's other companions at Saqifah. Shia Islam or Shi'ism is the second-largest branch of Islam.

For other uses, see Shia (disambiguation).
