Shi'i Islam, which significantly values and revolves around the 12 spiritual leaders called Imams, differs significantly from the beliefs of Sunni Islam. Unlike Sunni Islam, "Messianism is an essential part of religious belief and practice for almost all Shi'a Muslims. "  Shi'i Islam believes that the last Imam will return again, with the return of Jesus. According to religious scholar Mona Sidique, "Shi'is are acutely aware of the existence everywhere of the twelfth Imam, who disappeared in 874. Shi'i piety teaches that the hidden Imam will return with Jesus Christ to set up the messianic kingdom before the final Judgement Day, when all humanity will stand before God. There is some controversy as to the identity of this imam. There are sources that underscore how the Shia sect agrees with the Jews and Christians that Imam Mehdi (al-Mahdi) is another name for Elijah, whose return prior to the arrival of the Messiah was prophesied in the Old Testament.  On the other hand, there is also belief from among Shia and Sunni adherents that the imam will be Muhammad.