Imam Jafar Al-Sadeq (AS) was not only the teacher of Shia Muslims, but he was also the teacher of Sunni Muslims of his time. Shia and Sunni Muslims were all brothers in his time and never had a problem.
This is according to Dr. Noor Ahmed Shahtaz, Sunni Hanafi professor at University of Karachi in Pakistan, who, in an interview elaborated on Imam Sadeq's (AS) character and contributions.
Here are some excerpts from the interview minus the questions:
In Sunni school of thought the Imam (AS) is regarded as the leader of Sunni scholars in various fields including Islamic education and jurisprudence. Imam Sadeq (AS) contributed significantly to promoting the genuine teachings of the Holy Quran and always referred to the Divine Book and sentences narrated by the Holy Prophet (PBUH) in his classes to answer pupils' questions.
Imam Sadeq (AS) also played a crucial role in developing scientific teachings of Islam and the Quran. The Imam (AS) made all-out efforts in order to foster unity and solidarity among Muslims and in fact he revived the teachings of Islam by educating 4000 students.
Muslims should not allow Western countries to foment discord among them and ought to maintain their unity in the face of enemies' plots.
It is worth mentioning that Dr. Noor Ahmed Shahtaz holds PHD degree in Islamic Studies. He is also the director of Sheikh Zayed Islamic Research Center in University of Karachi in Pakistan.