The study and knowledge of the Qur'an is essential for every learned person as well as for all faithful believers. It is specially essential for those scholars who are interested in the study of man and society, since this book has been effectively instrumental not only in moulding the destinies of Islamic societies, but also in shaping the destiny of the human race as a whole.
A brief glance over history would be enough to provide sufficient proof of the claim that there has been no such book that has ever influenced human societies to the magnitude of the Qur'an. It is for the same reason that the Qur'an automatically steps into the precincts of sociological discussions, and becomes the elemental constituent of the subjects of research in this discipline. This means that any deep study and profound research in the field of world history of the last fourteen hundred years, is impossible without the knowledge of the Qur'an.
The study of the Qur'an is essential for every committed Muslim, since it is the main source and foundation of the religious thought and faith. Whatsoever gives meaning, essence and sanctity to his existence lies in the Holy Qur'an.
The Qur'an is not just like other religious books which are content to discuss the problems of existence of God and creation in cryptic tones, or like those which merely convey a series of simple moral advice and counsels, so that those who believe in them are hopelessly left to search for guidance in other sources. Unlike such books the Qur'an formulates the tenets of faith besides communicating the ideas and views that are essential for a man of faith and belief.
Similarly, it also lays down the principles of moral and ethical values for the purpose of social and familial existence. It leaves the job of explanation, interpretation, and occasionally that of ijtihad and application of principles (usul) to secondary matters (furu') to be dealt with through ijtihad and sunnah.
Accordingly, utilization of any other source depends on the prior knowledge of the Qur'an. The Qur'an is the criterion and standard for judging all other sources. We should judge hadith and sunnah in the light of the Qur'an. We can accept it only when it is in accordance with the Qur'an, otherwise we do not accept it.
There are four more books that come after the Qur'an, and are regarded as the most sacred and the most authentic sources (by the Shi'ah Muslims). They are: Al-Kafi, Man la yahduruhu al-faqih, Tahdhib, and Istibsar. There are also other sources like the Nahj al-Balaghah, and the prayers of al-Sahifah al-Sajjadiyyah. All these books are secondary to the Qur'an, and their authenticity of source is not so absolute as that of the Qur'an.
A hadith from al-Kafi is as trustworthy as it may be in conformity with the Qur'an, and reliable so far as its words comply with the teachings of the Qur'an and do not go against it. The Prophet (S) and the infallible Imams have said that their traditions should be checked in the light of the Qur'an; if they do not coincide with the words of the Qur'an, they should be regarded as false and fake, and as being wrongfully attributed to them; since they have not said anything that can go against the Qur'anic teachings.