As is known, Ja'far Sadiq (جعفر الصادق) is one of the imams/scholars and Faqeehs of the Aalul Bait, and a number of scholars took from him like Malik and Abu Hanifah and others. But like many of the scholars, his fiqh was not fully written down, and following him as a Madhab wasn't really famous. But as is also known, the Shi'a have a Madhab known as the Ja'fari Madhab, so my question is about its history, What is its origin? And how was it started?
Ja'fari Madhab is the School of Jurisprudence of most Shiites. It is derived from the name of Imam Ja'far ibn Muhammad Sadiq, the 6th Shiite Infallible authority (Imam), a descendent, and in fact a member, of the Prophet’s Ahl al-Bayt or Household according to the Shiite concept of the term.
Ja'fari School is thus distinguished from Sunni School in its recognition of the infallible scholarly and spiritual authority of Ahl al-Bayt which are viewed as the only legitimate inheritors and interpreters of the Prophet Muhammad's religion. Ja'fari School, therefore, draws upon traditions from all legitimate successors of Prophet Muhammad, a total of 12 Imams.
But as for why the Shiite Jurisprudence came to be associated with the name of the 6th Imam, a little history is necessary.
Ahl al-Bayt came under vicious restriction and intrusive monitoring of Umayyad and Abbasid caliphs who saw them as a delegitimizing challenge to their undeserved power. Their teachings were also banned by the ruling caliphs and their followers could even risk being punished by death. These repressive measures remarkably reduced Ahl al-Bayt recognition by Islamic Ummah and those seeking Islamic knowledge.
Yet, the transitional period between the Umayyad and Abbasid dynasties, and also Abbasid's early (although mainly pretentious) policy of "respect for Ahl al-Bayt" provided an extent of freedom and public availability that had been (and would have remained) unprecedented throughout the lives of the 12 Imams.
As the Abbasids and the Umayyads were busy fighting each other, Imam Sadiq seized the opportunity to promote Prophet's Wisdom as inherited by Ahl Al-Bayt. As never before a member of Ahl al-Bayt had risen to such fame and recognition as of Imam Sadiq to promote Islamic Jurisprudence according to Ahl al-Bayt’s interpretation, the School of Shiite Jurisprudence became as much associated with the name of Imam Jafar Sadiq as it was established by him.
It is noteworthy that according to Shiism, there is no difference between wisdom of the Prophet (SAWW) and that of all members of Ahl al-Bayt including Imam Sadiq a.s., yet never in decades to come, people would so widely turn to Ahl al-Bayt as best representatives of Islam and the Prophet's Sunnah as they did during Imam Sadiq's life.
Ja'fari Madhab has a name and content. Although the name may not be properly suitable for this school of jurisprudence, Shiite School of Jurisprudence was first officially established under this title and applied to people following it. Shiites argue that the title can be basically understood as meaning “School of Prophet Muhammad and his Ahl al-Bayt”.
For a thorough understanding of the origin and content of this Madhab a historical background is essential. Years before birth of Prophet Muhammad (SAWW) there were two main rival tribes among arabs with rival ideologies. One was the monotheist Hashemites (i.e. forefathers of Prophet Muhammad (SAWW)) who followed the religion of Ibrahim a.s. (called Hanif religion). The other tribe was the Umayyads who were idol-worshiping pagans. They held 360 sacred idols inside Kaaba. Some of the idols are mentioned in Quran:
So have you considered al-Lat and al-'Uzza? (53:19)
When Prophet Muhammad SAWW emerged and introduced Islam, Umyyads led by Abu Sufyan viewed his message as a fundamental threat to their ideological and sociopolitical status. So they repeatedly tried to assassinate the Holy Prophet (SAWW) on many occasions including during various armed battles (such as Badr, Uhod, Trench wars), but they failed in all cases. The Prophet SAWW finally conquered Mecca which was the center of the pagans. Abu Sufyan who had fought the Prophet (SAWW) for 25 years immediately declared “I am Muslim!” to save his life, and the Prophet accepted his submission and refrained from executing him.
Abu Sufyan was the great chief of the pagan tribes. He held much wealth and power and had a widespread tribal support. But after his submission, nothing really changed. In fact his enmity against the Prophet only changed a covert one, a hypocrisy. Hypocrisy or concealed enmity was the only way he and his supporters could survive.
Over the next years, these two rival dynasties sought to establish their own rule. Umayyads, despite seeking a pagan reign over Islamic Ummah, had to cover up their intentions behind Islamic pretenses. The Hashemites as led by Prophet Muhammad genuinely sought an Islamic government according to the Islam. This rivalry last well after the death of the Prophet SAWW and Abu Sufyan. The leaders changed but the two opposite ideologies kept fighting. Now the Prophet’s Ahl al-Bayt had to shoulder the Prophetic mission but they were denied the pivotal role they fully deserved and fully needed (and in fact were vested with by the Prophet) to carry on the Prophet’s mission in the Ummah and this was to be blamed on the self-proclaimed supremacy of much less worthy companions of the Prophet.
Some 40 years later, after the Hashemites had become completely marginalized among the Ummah by early caliphs, Muaviyeh the son of Abu Sufyan (and later his son Yazid) got the opportunity to fulfill their pagan tribalistic dreams they'd been stealthily and patiently pursuing for decades: taking over the Prophet’s Ummah and government and diverting it towards their own satanic desires. Once in power they engaged in fabricating their own version of Islam by hiring corrupt scholars and ordering them to forge hadiths.
The substantially irreconcilable struggle between the villainous Umayads and the righteous Hashemites culminated in the battle of Karbala where Prophet Muhammad's Ahl al-Bayt were mercilessly slaughtered in the hands of Yazdi's army.
About half a century later, the Umayyads were superseded by Abbasids, but Abbasids were not much different in treating Ahl al-bayt than the Umayyads. They kept Ahl al-Bayt away from people and power. They feared the spiritual status of Ahl al-Bayt as they could encourage people to revolt against Abassids’ undeserved power. Therefore the Abbasids had to resort to propaganda and other tactics to isolate Ahl al-Bayt. But soon they'd learn that they had no option but to keep the Imams in prison and ultimately assassinate them to remove the ideological and political challenge Ahl al-Bayt posed to their legitimacy and power.
They want to extinguish the light of Allah with their mouths, but Allah will perfect His light, although the disbelievers dislike it. (61:8)
Hadiths suggest that prophets do not leave gold and silver as heritage but wisdom, and Prophet Muhammad is no exception. So there should always be a person who receives this divinely revealed wisdom and expound upon it for later generations. The sign of such a person is that he should never say I do not know. But a glance at Ahl al-Bayt life as the only worthy candidates of this great task reveals that they had never the freedom to propagate the true wisdom of the Prophet (SAWW) that was passed to them as heritage.
Yet, despite all repressions and restrictions imposed on them they did manage to train many meritorious students outside the reach of tyrant’s radar. They stealthily trained Islamic scholars purporting to be their servants. For example Imam Ali a.s. taught part of the knowledge of tafseer(interpretation) to Ibn Abbas. In fact those who lived in Ahl al-Bayt’s homes as their servants were trained according to the inherited wisdom of the Prophet SAWW. They would be, then, sent to regions across the Islamic domain such as Iran for disseminating genuine Islamic Wisdom among the Ummah.
Among members of Ahl al-Bayt who all lived under close monitoring the ruling government’s agents, Imam Sadiq a.s. was an exception. When the Umayyad dynasty was on the decline and Abbasids were fighting for the throne, Imam Sadiq a.s. was as a result relieved of historical political restriction that had long subjected his fathers. He made the best advantage of the opportunity to train thousands of Islamic students according to the genuine Prophet’s Sunnah. His students wrote down his lectures until they filled 400 volumes of transcripts. These 400 books (The 400 Principles, اصول اربعائه) formed the basis of Jafari Fiqh and were passed to next scholars. Some of these books survived and some were destroyed in the course of history. But the knowledge could pass to later generations up today.
This open atmosphere though didn’t last long and the restrictive political clout was soon restored once Abbasids established their power and Ahl al-Bayt went under close political monitoring again. For example Imam Hassan al-Askari a.s. (the father of Imam Mahdi a.s.) spent nearly all of his life in custody in an Abbasid garrison, a fact that is reflected in his nickname Askari which refers to, Askar, a Samarra military district where he was kept.
This persistent political repression and isolation of Ahl al-Bayt by corrupt caliphs was the main reason Ja’fari Madhab or Shiite School of Thought remained widely unknown and unpopular among general masses. Yet, thanks to steadfastness and unwavering commitment of a minority of bright and faithful followers of Ahl al-Bayt who were willing to pay with their lives for guarding and upholding Prophet’s True Sunnah, the Islamic vast wisdom as represented by its most worthy representatives was successfully passed to us which covers various fields, such as theology, cosmology, gnostic meanings of the Quran, ethics, history, medicine, etc.
(documentary evidence for various parts of this narrative is available and can be shared in the form of answers to new separate questions)