Ja'far al-Sadiq gives his name to the Ja'fari school (madhhab), the system of law followed by Twelver Shia Muslims and standing alongside the four Sunni schools; in this sense he is regarded as its founder, the Imam whose teaching became the basis of Shia fiqh. Distinctive features later associated with the school, such as the recognition of temporary marriage (mut'a) and particular rules of inheritance and ritual, are traced in the tradition to his instruction.
More fundamentally, his period is associated with the maturing of the Shia doctrine of the Imamate itself: the teaching that after the Prophet, authority to guide the community and to interpret the faith rests with a divinely designated line of Imams from the house of Ali, each appointed by explicit designation (nass) from the last, and each possessing a God-given knowledge of religion. The elaboration of this doctrine of designated, infallible guidance is bound up with the figure of Ja'far and the circle around him.
The great early Shia collections of tradition, above all the Kitab al-Kafi of al-Kulayni, preserve a large body of teaching reported from him on law, theology and the conduct of life, making him one of the most frequently cited authorities in Shia literature.