Occultation (Arabic: غَيْبَة, ghaybah) in Shia Islam refers to a belief that the messianic figure known as the Mahdi, an infallible male descendant of the Islamic Prophet Muhammad, has already been born and subsequently went into hiding, from which he will one day emerge and establish a global Islamic state.
The groups that do believe in it differ on the succession of the Imamah, and therefore which individual is in occultation, with the largest Shia branch holding it is Hujjat-Allah al-Mahdi, the twelfth Imam. The Hidden Imam is still considered to be the "Imam of the Era", to hold authority over the community, and to guide and protect individuals and the Shia community.
In Twelver Shia Islam, the largest branch of the Shia faith, twelfth imam, Hujjat-Allah al-Mahdi, went into the Minor Occultation in 874, in which he communicated to his Shias through the Four Deputies.
The Minor Occultation (Arabic: ٱلْغَيْبَة ٱلصُّغْرَىٰ, al-Ghaybah aṣ-Ṣughrā) refers to the period when the Twelver Shia believe the Imam still maintained contact with his followers via deputies (Arabic: ٱلنُّوَّاب ٱلْأَرْبَعَة, an-Nuwwāb al-ʾArbaʿa). During this period, lasting from 874 to 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, the eleventh Imam, Hasan al-Askari, 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, known as The Four Deputies, became mediators between the Imam and his followers known as Sufara until the year 941. This period is considered by Twelvers to be the first, or minor occultation.
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 Imams had, on occasion, met with their followers from behind a curtain.
System of the DeputyEdit
According to Twelvers, under the critical situation caused by Abbasids, Ja'far al-Sadiq was the first to base the underground system of communication in the Shia community. By the time of Muhammad al-Jawad, the agents took the tactic of al-Taqiyya to take part in government. From the time of Ali al-Ridha, 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. Shia tradition holds that the Four Deputies acted in succession:
- Uthman ibn Sa'id al-Asadi (873–874): He was the first deputy appointed by 12th Imam who governed for one year;
- Abu Jafar Muhammad ibn Uthman (874–916): He was the second deputy appointed by 12th Imam for forty-two years;
- Abu al-Qasim al-Husayn ibn Ruh al-Nawbakhti (916–937): He was the third deputy appointed by Al-Mahdi for twenty-one years;
- Abu al-Hasan Ali ibn Muhammad al-Samarri (937–940): He was the last deputy of 12th Imam for three years.
The fourth deputy died six days later, and Twelvers continue to await the reappearance of the Mahdi. In the same year, many notable Shia scholars such as Ali ibn Babawayh Qummi and Muhammad ibn Ya'qub al-Kulayni, the learned compiler of al-Kāfī, also died. 
The Major Occultation (Arabic: ٱلْغَيْبَة ٱلْكُبْرَىٰ, al-Ghaybah al-Kubrā) 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 Hujjat-Allah 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 narrations, for they are my Hujjah (proof) unto yo! and I am Allah's Hujjah unto them."
Before the rise of the Fatimid Caliphate, a small group of Ismailis, the Qarmatians, believed that Muhammad ibn Isma'il had gone into Occultation and were called "Seveners" to reflect their belief in only seven Imams, Muhammad's father Isma'il being the last until his return. The Qarmatians accepted a Persian prisoner by the name of Abu'l-Fadl al-Isfahani, who claimed to be the descendant of emperors, as the returned Muhammad ibn Isma'il and also as the Mahdi. The Qarmatians 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 qibla 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.
According to Tayyibi Isma'ilism, during the Occultation of the twenty-first imam, at-Tayyib Abu'l-Qasim, a Da'i al-Mutlaq "Unrestricted Missionary", maintains contact with him. The several branches of the Musta'li Shia differ on who the current Da'i al-Mutlaq is.
The Nizari Isma'ili believe that there is no Occultation at all, that the Aga Khan IV is the 49th Imam of the Time. They believe that the Imam's authority is no different from the authority of Ali, the first Imam; he currently provides guidance to Nizaris on worldly and spiritual matters.
According to Waqifi Sevener Shias, there are reasons for the occultation: Imam not being proud of himself and continue to examine world events and make evaluations, only to be loyal and reverent to the Lord and Allah, which are also in Occultation, the other Muslims sects are to be judged by the Mahdi through the guidance of the Lord and Allah.
The Druze believe the imam Al-Hakim bi-Amr Allah went 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.
The Zaidi believe that there is no occultation at all and that any descendant of Hasan or Husayn could become Imam. The Imam must rise up against oppression and injustice and rule as a visible and just ruler.
Some scholars, including Bernard Lewis, 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), Muhammad al-Nafs al-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 Shias in order to maintain the coherence and continuity of the Shia movement after the death of the 11th Imam, Hasan al-Askari. Bahá'ís believe that Sayyid Ali Muhammad Shirazi, known as the Báb, is the promised Twelfth Imam, the Mahdi, who had already made his advent and fulfilled all the prophecies. The Báb was publicly executed on July 9, 1850 A.D. The Bab repeatedly talked about a messianic figure in his writings called He Whom God shall make manifest that would appear after him and would be the origin of all divine attributes. The prediction is widely recognized as being fulfilled by Baháʼu'lláh, the founder of the Baháʼí Faith.
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