Beliefs

There are numerous Qur'anic references substantiating the need for imams:

Verily your guardian is God then His Prophet and all believers; they are Allah's contingents, they are those who will remain triumphant (against infidels and non-believers). 46

Obey your God and His Prophet and those in authority from among you. 47

These two verse from the Holy Qur'an highlight the need for an imam in Islam. It is stated that one must obey one's Imam as one would obey one's God and one's Prophet. The significance of the imams can also be traced from several ahadeeth compiled in the various Shi'ah and Sunni books where it is stated again and again that Prophet Muhammad (saws) said: "There will be twelve leaders after me." 48
With regards to the question about the Twelfth Imam, there are several hints in the Qur'an and ahadeeth describing Al-Mahdi (as). Even Divine books like the Taurah and Psalms have mentioned Al-Mahdi (as) as the Imam of the Age, as stated in the Qur'an:

And we have said in the Psalms and the Torah that My righteous surely will inherit the Earth. 49

On another occasion Allah (swt) has referred to Himself saying:

He is the One Who sent His Prophet with guidance and true religion, superior to all the previous religions. 50

The sending of Al-Mahdi (as) and his appearance is a commitment Almighty God (swt) has made with mankind and the time will come when he will appear, as prophesized Qur'anic expressions such as: "??????? ???????", which means that the victorious are solely the pious, also refer to Al-Mahdi's (as) advent in this world. 51

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46. Qur'an 5:56

47. Qur'an 4:59

48. Narrated from Imam Al-Ridha, in Bihar al-Anwar 36:41, Vol.5, Section 4, p.136

49. Qur'an 21:105

50. Qur'an 9:33

51. Araki, Mohsin, Ideological and Jurisprudential Frontiers of Islam, pp.17-18, Book Extra, London, 1999

Adapted from the book: "The Awaited Saviour; Questions and Answers"