His title Zain-al-Abideen was granted to him by the Holy Prophet (pbuh) himself who mentioned that on the Day of Judgement a call for Zain-al-Abideen will be made and my son Ali bin al-Hussein (AS) will respond to the call saying "Labbaik". His other title, Sayyid-ul-Sajideen, was given because of his devotion to prayers. He would pray for long durations especially during the nights and would pray a lot of prayers of gratitude or the Namaz-e-Shukrana.
Imam Sajjad (AS) spent the first two years of his life under the care of his grandfather Imam Ali (AS) and after his death in 50 AH, he was brought up under the care of Imam Hassan (AS), second Shia Imam. Imam Sajjad (AS) was married to Bibi Fatima (AS) - daughter of Imam Hassan (AS). Imam Hassan (AS) was martyred in 50 AH and the Imamate of Imam Hussain (AS) started which terminated on 10th of Muharram 61 AH from where the Imamate of Imam Sajjad (AS) began.
The Imam kept his life very personal and preferred to stay in a town close to Medina from where he would preach the true religion of Allah quietly and with character. His character and preaching inspired a large number of people especially in the vicinity of Medina and Mecca. Slowly the tyrant rulers of his time began to realize the dangers that they faced from the Imam's preaching and character. His period of Imamate was full of tyrant rulers such as Yazid and Marwan bin-al-Hakam, Abdul Malik bin Marwan and Walid bin Abdul Malik. Amid this growing threat, Walid decided to poison and kill the Imam and finally he succeeded in his ulterior motives and Imam Sajjad (AS) was poisoned by the governor of Medina and martyred on 25th of Muharram 95 AH (713 AD).
He familiarized people with the lofty ideals of Islam through supplications to God Almighty. Here is a piece of advice from the Imam to the people quoted by his disciple Abu Hamza Thomali:
Avoid association with the sinful and wrongdoers; avoid cooperation with the oppressive and contact with the corrupt people. Keep distance from them and beware that whoever opposes the divine leaders and follows a religion other than that of God he or she will go to hell.
The 4th Imam kept the memory of the tragedy of Karbala alive. Whenever he would drink water, he would burst into tears in recalling the thirst of his father and the martyrs of Karbala. One day when someone asked him how his grief would come to its end, he replied: Prophet Jacob, who had 12 sons, on losing only one son, wept so bitterly that he became blind; but I have witnessed the killing of my father, my brothers, my uncles and 17 of my kinsmen, whose dismembered bodies were spread on the ground; so how can my sorry end. The personality of Imam Zain al-Abedin (AS) unmasked Omayyad hypocrisy, as he revealed their crimes against humanity by saying: We were treated like the Israelites in the time of the Pharaohs; they killed our men and let our women alive. The Arabs despite acknowledging the superiority of the Quraish subjected us to utmost atrocities, despite the fact that we were the children of the noblest of them, Prophet Mohammad (pbuh).
It is narrated that after Imam Sajjad (AS) returned to Medina from Karbala -- maybe ten or eleven months passed between the time this caravan left Medina and returned -- a person visited him and said: "Oh! Son of the Messenger of Allah, you see, you went and what happened?” He was right; the caravan had left while Imam Hussein (AS), the bright sun of Ahl al-Bait, so much loved by the Messenger of Allah, was among them, leading them. The daughter of Imam Ali (AS) had left while she was dignified and honored. The children of Imam Ali (AS) --Abbas and others-- the children of Imam Hassan (AS), prominent and talented youth of Bani Hashem had all left with this caravan. But the caravan returned with only one man -- Imam Sajjad (AS), with the women gone through sufferings of captivity and loss of their loved ones. Imam Hussein (AS), Ali Akbar (AS), even the baby were now gone. Imam Sajjad (AS) answered that man, stating: "Imagine what would have happened if we had not gone."
Yes! If they had not gone, their bodies might have survived, but the truth would have died. The soul would have died out. Consciences would have been shattered. Logic and reason would have been condemned throughout history, and even the name of Islam would not have remained.