Ja’far ibn Muhammad (AS) known as the Ja’far as-Sadiq (AS) is the sixth Shiite Imam. Imam Sadiq (AS) was born on 17 Rabi al-Avval 83th (702 CE) in Medina. His father, the previous Imam, was named Muhammad al-Baqir (AS) and his mother was called Umi-Farvah. After his father, Imam Sadiq (AS) became the Imam of the Shias and his leadership lasted for about 34 years. The era of Imam Sadiq (AS) was the time of instability and wars, since the Umayyad dynasty were weakened so much due to the battles they had in the Islamic territory. In addition , Abbasid dynasty were trying to topple the Umayyad government and reign over the whole Islamic world. The weakness of the rulers gave Imam Baqir (AS) and Imam Sadiq (AS) a perfect opportunity to spread the teachings of Shiism and train so many Shia scholars. At last, Imam Sadiq (AS) was martyred by the order of the Mansur (an Abbasid Caliph) on 148 H (765 CE). 

What does “Sadiq” mean?

The word “Sadiq” means an honest person who does not tell lies. 
Once, Imam Sajjad (AS), the grandfather of Imam Sadiq (AS) was asked: “Who is the Imam after you?”
“My son Muhammad (AS) who opens knowledge and after him his son Jafar who is called in the skies as ‘Sadiq’” Imam Sajjad (AS) replied. 
He was asked once more: “Why out of you all, he is called “Sadiq” while you all are honest?”
Imam Sajjad (AS) said: “Because the Prophet (PBUH & HP) said: ‘Name my son Jafar ibn Muhammd as “Sadiq” because there will be another Jafar in the future who falsely claims to be an Imam and he is named “liar” by Allah’” (On the contrary, Jafar ibn Muhammad is honest because he is a true Imam). (1)
The liar Jafar mentioned in the hadith is the son of Imam Hadi (AS), the eleventh Imam. 

The knowledge of Imam Sadiq (AS)

All Shiite Imams were known for their abundant knowledge in their time; but among all, Imam Baqir (AS) and Imam Sadiq (AS) were different. Due to the fact that Umayyad dynasty were weak and unstable in those times, Imam Baqir (AS) and Imam Sadiq (AS) had this chance to unveil the abundant knowledge of the Prophet’s household. 
About 4000 people learned from Imam Sadiq (AS) and narrated hadiths from him . (2)
Amr ibn Abi al-Mighdam, a Sunni scholar, says: 
“Every time I looked at the face of Jafar ibn Muhammad (AS), it would make me sure that he is a descendant of the prophets. Once, I saw him in Jamrah (a place in Mecca where the pilgrims gather) and he was telling people: ‘Ask me! Ask me!’” (3)
Hassan ibn Ali al-Vasha, a Shiite hadith narrator, says:
I saw about 900 sheikhs who all would narrate hadiths from Jafar ibn Muhammad (AS) (4).
Once Abu Hanifah, one of the most knowledgeable and popular Imams of the Sunnis, was asked: “Who do you think is the most knowledgeable person in religious fields?” he said: “I do not know anyone more knowledgeable than Jafar ibn Muhammad (AS)” (5).
Imam Sadiq (AS) is mostly known for his knowledge in Islamic laws and because of this, the Shiite school of law is called “Jafari School”. The hadiths narrated from Imam Baqir (AS) and Imam Sadiq (AS) are more than all the hadiths narrated from the Prophet (PBUH & HP) and the other Imams all combined together.

The moral attributes of Imam Sadiq (AS)

In addition to his wisdom and knowledge, Imam Sadiq (AS) was also known for his significant moral qualities. 
Muhammad ibn Talha says: 
“Imam Sadiq (AS) was the greatest person among the Prophet’s household and he was so knowledgeable and he would pray and recite the Quran so much and he was an ascetic person.” (6)
Malik ibn Anass, one of the Sunni school’s head, says: 
Every time I met him, he was in one of these three moods: either praying, fasting or whispering prayers (7).
Hisham ibn Salim, a famous apprentice of Imam Sadiq (AS) says:
Abu-Abdillah (Imam Sadiq (AS)) would always take a bag full of bread, meat and money in the darkness of the nights and go to the house of the needy ones in Medina and give it to them while they didn’t know him. Then, when he died, the people weren’t given those goods anymore and from that, they found out about his identity. (8)

Resources 

1.  Al-Khara’ij, ar-Ravandi, vol.1, pg.268
2.   Al-Irshad, Sheikh Mufid, vol.2, pg.179
3.    Siar A’lam an-Nubalah, Zahabi, vol.6, pg.257
4.    Rijal al-Kashi, vol.1, pg.138-139
5.   Siar A’lam an-Nubalah, Zahabi, vol.6, pg.257
6.    Kash al-Qummah, Irbeli, vol.2, pg.691
7.  Bihar al-Anvar, Majlesi, vol.47, pg.16
8.   Kafi, Koleini, vol.4, pg.8
    

Imam Jawad (AS) is the ninth Shiite Imam. His father is Imam Reza (AS), and his mother’s name is  Sabikah. His original name is “Muhammad,” but his epithet is “Jawad,” meaning “a very generous person.” He was born on 195 H (810 AC) in Medina. (1)

Generally, Shiite Imams carried prestige among their tribe and were respected by everyone. Many Shias believed that Imams were merely knowledgeable people who gained their knowledge only from their ancestors and they were not extraordinary. However, the imamate of Imam Jawad (AS) made all the Shias think differently and change their minds. He was only eight years old, when his father, Imam Reza (AS), was martyred. According to hadiths, the earth cannot be without an Imam. Accordingly, Imam Reza (AS) said:
The earth cannot be without any of us, Imams, whether he is known by the people or hidden from them. (2)

Shias didn’t know who would be their leader after Imam Reza (AS). But, he introduced Imam Jawad (AS) as his successor (3) and showed the Shias that “Imam” was not an ordinary person and he could be the leader of the whole Shias despite being young. 

Knowledge is a sign of imamate 

The hadiths suggest various signs for Imams so that we can differentiate between the truly chosen Imams by Allah and impostors. One of the most significant signs is that he is the most knowledgeable person of his time. 

Prophet Muhammad (PBUH & HP) said:
Do not try to teach something to Ahlul Bayt because they are far more knowledgeable than you. (4)
Regarding this, Imam Sadiq (AS) acknowledged:
Indeed Ali (AS) was a knowledgeable person whose knowledge will be passed to his successors and no knowledgeable person (an Imam) dies except to leave their knowledge to someone else. (5)
He also said:
The successor of an Imam attains the knowledge of the previous Imam in the last moment of his life. (6)

The debates of Imam Javad (AS) with religious scholars

Ma’mun was one of the Abbasid Caliphs who lived during the same period as Imam Reza (AS) and Imam Jawad (AS). During his caliphate, the Shia population increased dramatically which intimidated him. Due to this fact, he lied that Imam Reza (AS) was his successor and led people to believe that he was not an enemy of Shiite Imams. But at last, he killed Imam Reza (AS). After Imam Reza (AS), Imam Jawad (AS) was his most important enemy. He made Imam Jawad (AS) marry his daughter so that he could deceive people longer. 

Many of the Abbasid royalties were against Ma’mun’s decision, as they believed that the boy was too young and he wasn’t a knowledgeable person. Ma’mun proposed that they should test the knowledge of Imam Javad (AS), in order to show everyone how extraordinary he is. Imam Javad (AS) was only nine years old at that time. 

One of the most knowledgeable religious scholars in those times, Yahya ibn Aktham, volunteered to have a discussion and debate with Imam Javad (AS). 
During the discussion, Yahya ibn Aktham asked Imam Javad (AS) about a person who has killed an animal while making Hajj; what should he do? (During the Hajj ceremony, it is forbidden for the pilgrims to kill any creature).
Imam Jawad (AS) said that it depends: Did he kill it in Mecca or outside of it? Did he know about this law or not? Has he done it intentionally or unintentionally? Was he free or was he a slave? Was it his first time? Was it a bird or something else? Was it a small animal or a big one? Is he ashamed of what he did or not? Did he do it at night or during the day? Was it Umrah (a special type of Hajj) or regular Hajj? 
Yahya was so surprised that he couldn’t answer the twelve questions of Imam Jawad (AS), but instead, Imam Javad (AS) answered them one by one and explained the rules to everyone. 
Ma’mun told people that “Now do you see what I was telling you?” (7)
During another session, Yahya asked Imam Jawad (AS) about a hadith from the Prophet (PBUH & HP) that Sunni narrators had recorded. The hadith said “Once Gabriel came to the Prophet (PBUH & HP) and told him ‘O Muhammad! Allah told you to inform Abu-Bakr that I am satisfied with him. Is he satisfied as well?” Yahya questioned Imam Javad (AS) about the authenticity of this hadith.
Imam Jawad (AS) said, “I do not deny the privileges of Abu-Bakr … but this hadith is contrary to the verses of the Quran.” Imam Javad (AS) stated he wouldn’t deny the privileges of Abu-Bakr because he was surrounded by Sunnis and it was so dangerous for him to speak out against Abu-Bakr.
Then he said, “The holy Quran says 
‘Surely We have created man, and We know the promptings of his heart, and We are nearer to him than even his jugular vein.’ (50:16) 
Then why should Allah ask Abu-Bakr whether he is satisfied with him or not while Allah already knows it?”
Yahya referred to another hadith that claims Abu-Bakr and Umar on the earth are like Gabriel and Michael in the skies. 
Imam Javad (AS) said “This hadith cannot be accepted as well, because Gabriel and Michael never worshiped anyone except Allah, while Abu-Bakr and Umar spent most of their lives as polytheists and worshippers of idols.” 
Yahya added that Abu-Bakr and Umar are the lords of the elderly in Paradise.
Imam Javad (AS) stated that it is impossible as people in Paradise are all young not old. Umayyad dynasty made this fake hadith because they didn’t like the authentic one narrated by the Prophet (PBUH & HP) that reads, “Hassan (AS) and Hussain (AS) are the lords of young people in Paradise.” (8)
Yahya said it also has been narrated that Umar is the one who illuminates Paradise.
Imam Javad (AS) responded that it is also impossible. Do you think that Paradise is not illuminated by the Angles and Prophets like Adam (AS) and Muhammad (PBUH & HP) but rather by Umar?!
Yahya said it has been claimed that the angels talk with the tongue of Umar.
Imam Jawad (AS) said that he does not deny the privileges of Umar. However, Abu-Bakr was better than Umar as he said, “I have a Satan that sometimes tempts me, so if you see me deviating, stop me.” 
Yahya asked many other questions about Abu-Bakr and Umar, and Imam Javad (AS) answered them all. (9)   
Not only Imam Javad (AS) did not fail to answer the questions, but also, he posed more questions that even the most knowledgeable scholars felt incapable of disputing with Imam Javad (AS). These debates happened when Imam Javad (AS) was only 8 or 9 years old. 
As it might be apparent, Shiite Imams were extraordinarily knowledgeable, and their knowledge was given to them by God not by learning in classes and the like. They were the most knowledgeable individuals in their era regardless of their age.

Resources

1.  I’lam al-Vara, Sheikh at-Tabarsi, vol.2, pg.91
2.   Kamal ad-Din, Sheikh Saduq, vol.1, pg.202
3.  Al-Kafi, Sheikh Koleini, vol.1, pg.320
4. Mu’jam al-Kabir, at-Tabarani, vol.5, pg.166
5.  Al-Kafi, Sheikh Koleini, vol.1, pg.221
6.  Al-Kafi, Sheikh Koleini, vol.1, pg.275
7.  Al-Ih’tijaj, at-Tabarsi, vol.2, pg.444
8. Sunan al-Kubra, Nasa’i, vol.7, pg.460
9. Al-Ih’tijaj, at-Tabarsi, vol.2, pg.446-448