In 1969, he returned to Lebanon and cooperated with Musa al-Sadr for founding the Supreme Shiite Council of Lebanon. He was first vice-president. Shamseddine was elected as president of the Council in April 1994. Also, he was deputy of Musa al-Sadr organization. 
Advocate of coexistence
Shamseddine was a moderate cleric and advocate of Christian–Muslimcoexistence. About this matter, he stated that: “there is no Lebanon without its Christians and there is no Lebanon without its Muslims.”
He was supporter of military resistance against the Israeli troops in Lebanon and formed the “Total Civil Resistance Against Israel” group after Ashura 1983 to counter the Israeli invasion. He declared that Shia attacks against Israeli forces is a religious duty. 
For avoiding practice Tatbir among people, he established a blood bankin Najaf to donate blood on Ashura day to patients who need it. Every year on 10 Muharram of the Islamic calendar, people voluntarily gave blood in the blood bank Instead of practice Tatbir. Also, Mohammad Mahdi established the Islamic University.
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