|During the time of his imamate (95 – 114 AH) Imam Muhammad al-Baqir (PBUH) was instrumental in the elaboration of Shi`ite beliefs and principles. The large body of hadiths reported form the Imam on the principles of belief, fiqh (jurisprudence) and ethics, is an illustration of the movement towards the systematization of Shi`ite teachings which initiated during his imamate. After his death, the mainstream of the Shi`ites came to view Imam Ja`far al-Sadiq (PBUH) as its leader, a group which unlike the Zaydis believed that the appointment of an imam was effected through divine decree (nass jaliyy) and which therefore came to be known as the Imamis. The term Imamiyyah has historically denoted a wide spectrum of Shi`ite tendencies who after Imam al-Baqir (PBUH) were led by his son Imam al-Sadiq (PBUH) and the rest of Shi`ite imams (PBUT) and who experienced a tumultuous existence during the intellectual upheavals of the 2nd and 3rd centuries AH.
The imamate of Imam al-Sadiq (114 – 148 AH) coincided with a period of political transition which resulted in an atmosphere conducive to cultural activity. This was also the period of genesis of many Islamic sciences. Imam al-Sadiq (PBUH) carried through the movement of elaboration and systematization of Shi`ite ideas which had been set in motion by Imam al-Baqir (PBUH). The numerous surviving hadiths of the Imam, in addition to delineating the creedal positions of Shi`ism, deal with the theological issues of the day, especially those relating to imamate and the status of the imam. These include the notion of the muftarid al-ta`ah imam (the imam to whom obedience is obligatory), the fact that the earth is never without an imam, the impossibility of the simultaneous existence of two or more imams, the notion of nass (the appointment of Imam `Ali by the Holy Prophet (PBUT), as well as the necessity of the divine appointment of other imams) in imamate, and the unique transmission of imamate from one brother to the other in the case of Imams Hasan and Husayn (PBUT).
Imam al-Sadiq (PBUH) took an active part in the theological discussions of the day, such as those on the divine attributes, qadar (predestination) and the religious status of one guilty of commitment of a grave sin (kabirah). His overall approach to these issues was one of fairness and moderation. On the topic of divine attributes he opted for a middle position between the advocates of ta`til (denying all attributes to God) and tashbih (anthropomorphism), i.e. he asserted the eternity of the divine attributes, such as knowledge, while considering God as being above all corporeal qualities. The discussion of the pre-eternity or the created-ness of the Quran is a corollary of the subject of divine attributes about which the Imam took a middle position, i.e. that the Quran is muhaddath and uncreated.
On the long standing issue of qadar, the famous opinion of Imam al-Sadiq (PBUH), shared by other imams of the Ahl al-Bayt (PBUT), is one of the negation of both extremes and belief in a position between absolute freedom and absolute determinism (amran bayn al-amrayn). The Imam, along with the rest of the Infallible Imams (PBUT), sets forth a distinction between the two notions of mashiyyah and iradah, and by postulating two types of iradah (divine will) – i.e., hatm and `azm – elaborates the possibility of God commanding something that is not determined in His mashiyyah. Finally, on the widely disputed topic of the definition of faith, the Imam, once again, adopts a middle-of-the-road position. While he underscores the significance of action in the realization of faith and considers the committer of a kabirah as one who has forfeited his faith, he none the less distinguishes islam from iman (faith) and thus deems such a sinner as having remained within the bounds of Islam.
As an example of an exact theological discussion, reference may be made to the Imam’s , and his successors’, notion of “khlaq la min shya’” as a means of elaborating the notion of ibda` as an alternative to the common one of “khalq min la shay’”. In the coming centuries, the same concept came be adopted by major philosophers such as al-Farabi. Unlike the view held by some orientalists, the above notion – which came to find acceptance in many theologico-philosophical works of Muslims as well as in those of Christians and Jews, from the 4th century AH onwards – can be easily traced back to the early Islamic sources.
The followers of the branch of Shi`ism led by Imam al-Sadiq (PBUH) came to be known as the Ja`fariyyah. Sayyid Humayri (105 – 173 AH), a Kaysanite convert to Shi`ism, has thanked God, in one his poems, for this taja`fur (turning into a Ja`farite).