Right from the past till today, reason and experience have confirmed the issue of expertise. History talks about expertise in arts and science. The spread and expansion of sciences, arts and know-how is all the result of expertise and specialty in diverse branches. Reason too guides us to accept this fact that we must refer to the focal point of science and knowledge. Obviously, in acquiring knowledge on religion, one must return to the real focal point and fountainhead.
In this connection, the real pivot is the Qur’an and Ahl ul-bayt (a.s.) [who also possesses the knowledge of Qur’an]. It’s necessary to acquire all the sciences and learn the creeds, ethics and precepts from them. Aside from numerous reliable and widely transmitted traditions from them, they have explained and interpreted the fundamentals and principles that have been complied and explained in several books like:
Nahjul-Balagha: Sermons, maxims and letters of Amir-ul-Mu’mineen (a.s.)
Sahifa-Sajjadia: Invocations and prayers by Imam Sajjad (a.s.)
Usul, Furu and Rauzah Kafi: written by Seqat’ul Islam Kulaini.
La-Ya’zural-Faqih: written by Shaikh Saduq.
Al-Estibsar: written by Shaikh-ul-Ta’efa Abu Jafar Tousi.
Al-Tahzeeb: written by Shaikh-ul-Ta’efa Abu Jafar Tousi.
Tafsir by Ali-ibn-Abraham Qommi.
Tafsir by Muhammad-ibn-Masoud Aiyashi Samarqandi.
Tafsir by Furaat-ibn-Abraham-ibn-Furaat Kufi.
Basa’er al-Darejaat, by Muhammad-ibn-Hasan-ibn-Farrokh Saffar and tens of other books on hadith and tafsir
Thus, in matters of Usul and Furu, as well as, ethics one must refer to these books.
About the matter that we should seek knowledge from them, Imam Baqir (a.s.) says about the verse:
(“Then let man look to his food” : Chapter Abasa:24):“So it is necessary that man looks to his food”
Imam Sadiq (a.s.) too said,“It means man should look at his knowledge and to see from whom he is acquiring it?” 
Knowledge and understanding is the main spiritual food, which must be nourished from its main source.
About the fact that we must refer to the Imams (a.s.) and the narrators of their traditions, it is quoted from Hazrat Mahdi (a.t.f.s.) as saying:
“At the times of new events, refer to the narrators of our words and traditions (and acquire your knowledge from them that is supplied from the limpid fountainhead of Ahl’ul-bayt).”
In this connection, refer to Usul-e-Kafi, Kitab-ul-Hujjah.
Besides, our reasoning and innate intellect dictates that in gratitude for the All-Beneficent, and as a form of servitude, we must believe and remain steadfast about Oneness of Allah, the need for a prophet, the presence of the Day of Judgment, reckoning and divine justice and the existence of guardians of religion i.e., Imamate and Wilayat.
The Knowledge and sciences must be acquired from its focal point and fountainhead. That which lifts the curtain from the human nature and essence (Fitrat ) are the holy verses of Qur’an, and the explanation given by its interpreters. Numerous verses guide us to those scholars. The holy prophet (s.a.w.a.) has determined and introduced them. He has given them the knowledge of the Qur’an, and commanded the people to follow them.
Consequently, about the exposition and explanation of innate creed, the words and sayings of the immaculate leaders are Hujjat (divine proof). They are reliable leaders in matters of Fiqh and ethics. And the more closer a Faqih (or narrator) is to them, the more is his reliability.
To sum up, the Book and the Sunnah, in their true interpreted forms, are the best and the worthiest source for acquiring the genuine Islamic doctrines and their carriers (immaculate ones) are the most reliable ones for presenting and explaining them.