Poster inside the Muslim American Youth Academy, a private elementary school attached to the Islamic Center of America, the largest mosque in the United States (and a Twelver Shi'i one) located in Dearborn, Michigan.
In Twelver Shia Islam, the largest branch of the Shia faith, the twelfth imam, Muhammad al-Mahdi, went into Occultation in 873. The Occultation is split into the Minor Occultation and the Major Occultation.
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.
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:
Rest assured, no one has a special relationship with God. Whoever denies me is not from my (community) [there is no deputy after him]. 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.
YaleCourses: The Early Middle Ages, 284--1000 (HIST 210)
15. Islamic Conquests and Civil War