Kamāl al-Dīn wa Tamām al-Niʿma

Apart from him, one of the great scholastic (kalam) theologians of the Imamiyah sect has said: Most of our opponents ask us regarding such matters on this subject whereas they should know that to believe in the occultation of the Imam of the Age is based on the fact that we should first believe in the Imamate of his purified ancestors. And to believe in the Imamate of his purified ancestors based on the fact that we testify to the prophethood of Prophet Muhammad صلى الله عليه وعلى آله وسلم and accept his Imamate and leadership, because right from the beginning it is a related matter not a pure academic subject. 
Discussion in religious topics should be based on Quran and traditions as Allah, the Mighty and Sublime says: …then if you quarrel about anything, refer it to Allah and the Apostle… (4:59) Thus when Quran, traditions and logical arguments prove the correctness of a matter, our stand shall be most preferable. And we say that all the groups of Zaidiyyah and Imamiyah are unanimous that the Messenger of Allah صلى الله عليه وعلى آله وسلم said: “Indeed, I leave behind among you two weighty things; one is the Book of Allah and the other is my progeny, my Ahlul Bayt, and these two shall be my successors after me. And the two shall not separate from each other till they arrive to me at the Kauthar Pool.” 
This tradition is accepted as authentic by the both sects. On the basis of this it is necessary that along with Quran there should be a person from Progeny possessing perfect knowledge about the revelation and interpretation of Quran, and who informs the people about what Allah, the Mighty and Sublime implies. 
The Messenger of Allah صلى الله عليه وعلى آله وسلم used to inform the people about its (the Quran’s) meanings and interpretation. And his Marefat (divine recognition) should not be such that he should interpret the Quran through corollaries and deductions, just as the Marefat of the Holy Prophet صلى الله عليه وعلى آله وسلم was neither based on corollaries nor on deductions. Nor was it on the basis of the principles of language or the way people speak among themselves. He used to inform about the divine implication in such a way that as soon as he said something it became a proved matter for the people. In the same way it is necessary that the Prophet’s Progeny should have perfect recognition of Quran and insight. 
Allah, the Mighty and Sublime says describing the qualities of the Messenger of Allah صلى الله عليه وعلى آله وسلم thus: «Say: This is my way: I call to Allah, I and those who follow me being certain» (12:108) Thus his followers are from his family and his progeny and they are same and they shall explain the Book of Allah through their perfect knowledge and insight. And when the interpreter of Allah’s commands is not apparently visible, it would become obligatory on us that we should believe that there is always a person from the progeny of the Holy Prophet صلى الله عليه وعلى آله وسلم who possesses all the knowledge of meanings and interpretations of Quran, because the Prophet’s tradition mentions that it is imminent. 
Imamiyah scholars have said: Allah, the Mighty and Sublime said: Surely Allah chose Adam and Nuh and the descendants of Ibrahim and the descendants of Imran above the nations. Offspring, one of the other… (3:33-34) Thus in this verse, on the basis of generality it becomes necessary that from the progeny of Ibrahim عليه السلام till now and forever there will always be a chosen one. It is so because Allah, the Mighty and Sublime, in His Book, has divided the people into two groups. One is the group of prophets, messengers and caliphs and the other comprises those whom the Almighty Allah has commanded us to follow. Therefore till the time there exist people on the earth who need guidance and directions there has to be a chosen person from the progeny of Prophet Ibrahim عليه السلام. 
And it is also necessary that some descendants of Prophet Ibrahim عليه السلام be preferred over others as Allah, the Mighty and Sublime says: Offspring, one of the other… Moreover it is an established fact that the Messenger of Allah صلى الله عليه وعلى آله وسلم, Amirul Momineen عليه السلام and Hasan and Husain عليه السلام were the preferred ones from the progeny of Prophet Ibrahim عليه السلام. On the basis of this verse after Husain عليه السلام, it is necessary that the chosen ones should be from his descendants. And if they are not from his descendants the words of “Offspring, one of the other…” will not be proved true. And also that he should not be from their womb. Also that the Imamate was transferred from Imam Hasan عليه السلام to his brother Imam Husain عليه السلام. Therefore it is necessary that his successor should only be from his descendants. This is the actual implication of this verse: Offspring, one of the other; and Allah is Hearing, Knowing. (3:34) So this verse implies that which is stated in the Sunnah and traditions.