Official Baha'i history of the Babi Movement and the Baha'i Religion by Moojan Momen

The approach of the Muslim year 1260 (1844) was accompanied by a general rise in expectancy of the return of the Hidden Imam. This was because that year marked the one thousandth anniversary of the disappearance of the Twelfth Imam and the beginning of the period of Occultation. There were several indications in the Qur'an and the Traditions that the dispensation of Muhammad would be one thousand years long and thus the year 1260 was greatly anticipated throughout the Shi'i world. (according to the Baha'i sources viz. Amanat and Cole)

Sayyid 'Ali Muḥammad Shīrāzī (1819-50), who took the title the Bāb (the Gate), was, until the death of Sayyid Kazim Rashtī in 1843, closely associated with the Shaykhi School. Then, in 1844, he put forward a claim and gained many adherents, initially mostly from among the Shaykhi School. At first the Bab only appeared to be claiming to be the Gate to the Hidden Imam and his followers kept to the Islamic Shari'a. But in 1848 he advanced the claim of being the returned Twelfth Imam himself who had come to abrogate the Islamic dispensation and inaugurate a new prophetic cycle.

Developing the argument of the Shaykhi School, from the Babī viewpoint, just as the Hidden Imam existed in the world of Hūrqalya, the realm of archetypal images, so the return of the Twelfth Imam was not the return of the self-same physical body of the Imam but rather the advent of a man who in the realm of Hūrqalya is the archetypal figure of the Imam. Thus it was that the Shaykhi teachings paved the way for the Bab and it is doubtful if the Bāb would have attracted so many adherents if it had not been for the Shaykhi doctrines.

The Bāb was put to death by a firing squad in Tabriz in 1850. He had appointed as his successor Mirza Yahyā, Subh-i Azal, and had prophesied the advent of another messianic figure whom he called 'Him whom God shall make manifest'. Privately in 1863 and publicly in 1866, Mirza Husayn 'Ali (1817-1892), who took the title Baha'u'llāh (Glory of God), claimed to be this messianic figure foretold by the Bāb. The majority of Babīs became Baha'is. Bahā'u'llāh considerably expanded the scope of his appeal beyond the confines of Shi'i Iran by claiming to be the fulfilment of the messianic expectations of other religions such as Judaism, Christianity and Zoroastrianism

Baha'u'llāh was succeeded by his son 'Abbas Effendi (1844-1921), who took the title 'Abdu'l-Bahā (Servant of the Glory). He was given the position of authorised interpreter of Baha'u'llāh's teachings. He appointed his grandson, Shoghi Effendi (d. 1957), as Guardian of the Baha'i Faith. Since 1963 the religion has been administered by an elected body, the Universal House of Justice.

(An introduction to Shi'i Islam - The History and Doctrines of Twelver Shi'ism by Moojan Momen)