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The Occultation (Arabic: غيبة Ghaybah) in Shia Islam refers to a belief that the messianic figure, or Mahdi, who in Shi'i thought is an infallible male descendant of the founder of Islam, Muhammad, was born but disappeared, and will one day return and fill the world with justice and peace. Some Shi'is, such as the Zaidi and Nizari Ismaili, do not believe in the idea of the Occultation. The groups that do believe in it differ on the succession of the Imamate, and therefore which individual is in Occultation. The Hidden Imam is still considered to be the Imam of the Time, to hold authority over the community, and to guide and protect individuals and the Shi'i community.
In Twelver Shia Islam, the largest branch of the Shia faith, twelfth imam, Muhammad al-Mahdi, went into Occultation in 873. The Occultation is split into the Minor Occultation and the Major Occultation. According to Shia, there are reasons for the Occulration: Imam being afraid of his being killed, not to be under the oath of tyrant rulers, the Shia are to be examined by God.
The Minor Occultation (Ghaybat al-Sughra) refers to the period when the Twelver Shia believe the Imam still maintained contact with his followers via deputies (Arab. an-nuwāb al-arbaʻa). During this period, from 874-941, the deputies represented him and acted as agents between him and his followers.
Shia believe that in 873, after the death of his father al-`Askari, the eleventh Imam, the 12th Imam was hidden from the authorities of the `Abbasid caliphate as a precaution. His whereabouts were disclosed only to a select few. Four close associates of his father became mediators known as Saf’ir, or between the Imam and his followers, until the year 941. This period is considered by Twelvers to be the first, or Minor Occultation (al-Ghayba).
When believers faced difficulty, they would write their concerns and send them to his deputy. The deputy would receive the decision of the Imam, endorse it with his seal and signature, and return it to the concerned parties. The deputies also collected zakat and khums on his behalf. For the Shia, the idea of consulting a hidden Imam was not something new, because the two prior Shia Imams had, on occasion, met with their followers from behind a curtain.
System of the Deputy
According to Twelvers, under the critical situation caused by Abbasides, Ja'afar al-Sadiq was the first to base the underground system of communication in the Shia community. By the time of al-Javad, the agents took the tactic of al-Taqiyya to take part in government. From the time of al-Reza, the Imams were under the direct control of the authorities and the direct contact between the Imam and the community was disconnected. The situation led to the increase in the role of deputies. The deputies undertook the tasks of Imams to release them from the pressure of the Abbasids. Shi'a tradition holds that four deputies acted in succession:
- Uthman ibn Sa’id al-Asadi
- Abu Jafar Muhammad ibn Uthman
- Abul Qasim Husayn ibn Ruh al-Nawbakhti
- Abul Hasan Ali ibn Muhammad al-Samarri
In 941 (329 AH), the fourth deputy announced an order by Muhammad al-Mahdi that the deputy would soon die, the deputyship would end, and the Major Occultation would begin.
The fourth deputy died six days later, and twelvers continue to await the reappearance of the Mahdi. In the same year, many notable Shi'a scholars such as Ali ibn Babwayh Qummi and Muhammad ibn Yaqub Kulayni, the learned compiler of al-Kafi also died. 
The Major Occultation denotes the second, longer portion of the Occultation, which continues to the present day. Shia believe, based on the last Saf’ir's deathbed message, that the Twelfth Imam had decided not to appoint another deputy. Thus, al-Samarri's death marked the beginning of the second or Major Occultation. According to the last letter of Muhammad al-Mahdi to Ali ibn Muhammad al-Samarri,
from the day of your death [the last deputy] the period of my major occultation will begin. Henceforth, no one will see me, unless and until Allah makes me appear. My reappearance will take place after a very long time when people will have grown tired of waiting and those who are weak in their faith will say: What! Is he still alive?"
Rest assured, no one has a special relationship with God. Whoever denies me is not from my (community). The appearance of the Relief depends solely upon God. Therefore, those who propose a certain time for it are liars. As to the benefit of my existence in occultation, it is like the benefit of the sun behind the clouds where the eyes do not see it. - Kitab al-Kafi, Muhammad ibn Ya'qub al-Kulayni
With regard to advice for his followers during his absence, he is reported to have said: "Refer to the transmitters of our traditions, for they are my hujja (proof) unto you and I am God’s proof unto them."
Ismaili before the rise of the Fatimid Empire believed that Muhammad ibn Ismail had gone into Occultation, and were called Sevener to reflect their belief in only seven imams, Muhammad's father Ismail being the last until his return. The Qarmatian Sevener branch accepted a Persian prisoner by the name of Abu'l-Fadl al-Isfahani from Isfahan, who claimed to be the descendant of the Persian kings, as the returned Muhammad ibn Ismail  and also as their Mahdi. They rampaged violently across the Middle-East in the 10th century, climaxing their bloody campaign with the stealing of the Black Stone from the Kaaba in Mecca in 930 under Abu Tahir Al-Jannabi. After the arrival of the Mahdi they changed their qiblah from the Kaaba to the Zoroastrian-influenced fire. After their return of the Black Stone in 951 and defeat by the Abbasids in 976 they slowly faded out of history and no longer have any adherents.
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According to the Tayyebi branch of Mustaali Ismaili Shi'a Muslims, during the Occultation of the twenty-first imam, Taiyab abi al-Qasim, a Da'i al-Mutlaq, meaning unrestricted missionary, maintains contact with him. The several branches of the Mustaali differ on who the current Da'i al-Mutlaq is.
The Nizari Ismaili believe that there is no occultation at all, that H.H. Karim Aga Khan is the visible 49th Imam. They believe that the Imam's authority is no different from the authority of Imam Ali the first Imam; he currently provides guidance to Nizari Ismailis with worldly and spiritual matters.
The Druze believe the imam Al-Hakim bi-Amr Allah has gone into the Occultation after he disappeared in 1021 followed by the four founding Da'i of the Druze sect including Hamza ibn-'Ali ibn-Ahmad leaving the leadership to a fifth leader called Baha Al-Din. The Druze refused to acknowledge the successor of Al-Hakim as an Imam but accepted him as a Caliph. The faith further split from Ismailism as it developed unique doctrines which often classes it separately from both Ismailism and Islam, these include the belief that Al-Hakim bi-Amr Allah was God incarnate.
Some scholars, including Bernard Lewis also point out, that the idea of an Imam in occultation was not new in 873 but that it was a recurring factor in Shia history. Examples of this include the cases of Muhammad ibn al-Hanafiyyah (according to the Kaysanites Shia), Muhammad ibn Abdallah An-Nafs Az-Zakiyya, Musa al-Kadhim (according to the Waqifite Shia), Muhammad ibn Qasim (al-Alawi), Yahya ibn Umar and Muhammad ibn Ali al-Hadi.
In the Bahá'í Faith, which sees the Báb as fulfilling the Islamic prophecy of al-Mahdi, Bahá'u'lláh and `Abdu'l-Bahá considered the story of the Occultation of the twelfth imam in Twelver belief to have been a pious fraud conceived by a number of the leading Shí`ahs in order to maintain the coherence and continuity of the Shí`ah movement after the death of the 11th Imam, Hasan al-`Askarí. Bahá'ís believe that Sayyid `Alí Muhammad-i-Shírází, known as the Báb, is the promised Twelfth Imam, the Mahdi, who had already made his advent and fulfilled all the prophecies. The Shaykhi movement of the early 19th century claimed to have made preparations for the Mahdi. In 1848 the Báb and his followers began to teach more openly, and the Báb was publicly executed in July 9, 1850 A.D (twenty-eighth of Sha’bán, in the year 1266 A.H).
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