1. The Book of Allah in its written form makes the universal message of Islam fairly known to the Muslims/non-Muslims who can read Arabic with understanding. Exhorting the believers to reach a higher understanding, Allah expects them to ponder at the deeper meaning of His last message to mankind. At this higher level, Muslim scholars have put excellent efforts to elucidate the deeper meanings of the Qur'an's verses.
Voluminous books were written for what is called Tafseer, i.e. interpretation. When it comes to the meanings intended by Allah, they are inaccessible except to the chosen Imams. Both the Qur'an and the practice of previous messengers naturally lead to the belief in Imamate. None but the purified can comprehend the Qur'an that is in the sight of Allah. Therefore, at this highest level of understanding Ta'weel becomes necessary.
2. True, neither the name of the first nor the last, nor any other Imam is being mentioned in the Quran. But, the Qur'an infers that Imam Ali is the one who possesses the knowledge of the Book. It also refers the Muslims to those who know the Ta'weel - i.e. the actual meanings of the revelation as intended by Allah. The Muslims contemporary to the Prophet Muhammad unanimously agreed that Ahl-ul-Bayt, i.e. the household of the Prophet as being introduced in the verses of Tatheer and Mubahalah, are the ones whom Allah entrusted with His knowledge - especially as it pertains to the implementation of justice in man's life.
It has become Mutawatar in the science of Hadith that the Prophet, not only appointed Imam Ali as a Vicegerent, but also declared the Imamate of his grandsons Hassan and Hussein, and the 9 Imams from Hussein's progeny. It was only the will of Allah on which the Prophet acted. This was very similar to giving his only daughter Fatimah in marriage to his cousin. Thus, the appointed 12th Imam (Muhammad bin Al-Hassan) was being introduced by the Prophet himself long before his birth in 255 AH.
3. Similar to the persecuted Israelite prophets, the Prophet of Islam's household have endured unimaginable suffering. The same way the mother of Moses had to put her newborn in a box (see Ark of the Covenant below) and throw him in the Nile River, Imam Mahdi's father had to hide his son from the henchmen of the ruling Caliph. But, the Imam was properly introduced to a close circle of 40 disciples.
After the poisoning of his father in 260 AH, the Imam executed his duties through four special representatives for 69 years. Since 329 AH, the Imam has been in occultation waiting for Allah's permission to appear for the eventual implementation of justice. His mission is undoubtedly universal.
As a minor resurrection, the Imam's appearance represents Islam's revival. For the divine knowledge he inherited, he is a true partner of the Qur'an. Only he knows the Ta'weel of the confuted (Mutashabahat) verses. The Muhkamat verses are self-explanatory for the learned.
Quoting the Qur'an on the resurrection of a prophet 100 years after his death, and bringing back to life the youths of the Cave after 300 years, are proofs that the Middle resurrection is not a Shi'ah invention (as some ignorant call it). Raj'ah simply means the coming of specific persons back to life - by the leave of Allah.
Some of these are pious (like the above examples) and others could be ordinary witnesses (like the dead whom Moses smite by the tail of a slaughtered cow).
The ultimate judge is always Allah. The majority of the wrongdoers will undoubtedly be sentenced on the day of reckoning. However, the Qur'an declares that the ones who have persistently committed heinous crimes will have two deaths and two resurrections.
Imam Mahdi will bring such to the book, especially if they were the ones who laid the foundation of persecution of Ahl-ul-Bayt and subsequently caused the tragedy of Karbala.
source : www.tebyan.net