Where does the rule to assemble Councils come from?
The conciliar model comes from the Holy Scriptures, specifically from the example of the Apostles, who held a Council in Jerusalem (Act 15) under the presidency of James, the local bishop of Jerusalem. The canonical tradition made this pattern a standing rule of Church life, directing the bishops of each region to meet in synod regularly (Apostolic Canon 37; Nicaea I, canon 5; see Q070).
Councils help bishops and presbyters make weighty decisions for the life of the Church. The Lord himself gives to the decisions of the Church such authority that whoever disobeys them places himself outside her communion: “Amen, I tell you that whatever you bind on earth will have been bound in heaven, and whatever you loose on earth will have been loosed in heaven” (Mat 18:17–18).