Two parameters determine the hierarchy of religious authority among Shiites: learning and genealogical relations to the house of the Prophet. Both are connected in the belief in the transmission of special knowledge and blessing (baraka) through the bloodline. The ideal embodiment of both are the Imams, the last of whom is currently in Occultation. Since Occultation, different institutions have emerged in Shiite history that provide guidance, although from fallible sources, to the believers until the Imam returns. Sometimes, and despite the prominent narrative of suffering, such religious authorities have assumed considerable political authority.
Apart from the Imams themselves and the early scholars, the development of a specifically Shiite clergy probably started properly when the twelfth Imam went into Occultation. From these early days onward, religious authorities in the Islamic world were also in charge of certain financial matters due to special taxes and a portion of booty dedicated to God according to Islamic law. After the death of Hasan al-Askari, the eleventh Imam, individuals, some of whom were part of the Abbasid administration, collected taxes and acted as mediators with the Hidden Imam. Even though this institution of the 'ambassador' (safir) ended with its fourth representative in 941, later attempts to build a Shiite clergy that fulfilled similar functions were more successful. Well-developed religious institutions also existed among minorities within Shiism, most notably the Ismaili Fatimids.
Distinctive titles of Twelver Shiite clerics are Ayatollah (Arabic: sign of God) and Grand Ayatollah, high-ranking scholars who—in modern Iran—are often also descendants of Muhammad. The first who had this title was the Allama al-Hilli (1250-1325). This scholar also laid the theoretical foundations that allowed Shiite clergy from the 17th century onward to legitimize their authority. By subscribing to rationalist notions of Islamic legislation, al-Hilli put the legal scholars, endowed with reason, in the position of assessing independently the legal implications of political matters. While parallel ideas can be found in Sunni thought, the authority of Shiite legal scholars rests upon a more elaborate, exclusive, hierarchical, and ambitious basis. Key to this is the notion of taqlid, i.e., of following an authority. The muqallids (i.e., those who practice taqlid) are those who follow the muqallads, namely the scholars qualified to practice ijtihad (an independent effort in order to achieve a solution for a problem in Islamic law).
In Iran, the authority of the religious scholars rested not only on these theoretical foundations, but also on their social basis as prayer leaders in the villages and small towns and religious leaders in the larger cities. Their economic strength resulted from the benefits they received from pious endowments as well as from their connections with landowners. Since the Qajar period (1794-1925), a hierarchy divided the muqallids from simple legal scholars who were mainly in charge of applying existing rules, and these from high-ranking authorities who were capable of absolute ijtihad. From this emerged in the mid-19th century the concept of the marja al-taqlid. This supreme position as the 'source for imitation' was limited to one scholar also sometimes referred to as 'deputy of the Imam' (naib al-imam).
There is no established mechanism or open competition for this position, although there has been a silent agreement that—after the Iranian revolutionary leader Khomeini (d. 1989) and the Grand Ayatollah Abu Qasim al-Khoei (d. 1992)—the Iraqi Ali al-Sistani (born 1930 in Mashhad) is a marja al-taqlid, the recently deceased Lebanese Fadlallah having been another candidate.
In Sunnism too, religious scholars, especially those who are highly respected, can claim great authority. Unlike their Shiite counterparts, however, they are not organized in a fixed hierarchy. Likewise, Sunnis are flexible when they consult scholars, whereas Shiites have to subscribe to the views of one particular person. The great influence of the Shiite scholars in Iran is reflected in the concept of vilayat-e faqih, the rule of the (religious) scholar. The Islamic revolution in Iran also meant a substantial re-interpretation of the eschatological vision of Imami Shiism. Instead of waiting for the Hidden Imam to restore justice, as the then predominant quietist line had suggested, scholars were able to create an Islamic society in the here and now. Given the predominant position of Iranian Shiites and the Iranian state compared to other Shiite communities and states where Shiites live, the developments in Iran have had an important effect on non-Iranian Imami Shiites.