Why are the Ecumenical Councils important to all Christians, not merely Eastern Orthodox Christians? Also, what was the "Imperial Church"?
The Eastern Orthodox sometimes refer to themselves as the Church of the Seven Councils. They argue that the Roman Catholics have added to the original doctrine of the church as set out in the councils with Papal decrees and local councils, and the Protestants have subtracted from that body of doctrine with a sola scriptura doctrine. Meanwhile, the Orthodox have remained true to the faith as determined by the Bible and Councils, neither adding to it nor subtracting from it but keeping the tradition of the universal church intact.
The early church was fractured by disagreements over many fundamental questions, such as the nature of Jesus and of the Trinity. Christians even disagreed over which books belonged in the Bible. The Ecumenical Councils provided a way for Christians to establish doctrines on these issues. The Orthodox argue that the universality of the seven councils shows that their conclusions, and the creeds that were developed by them, are divinely inspired.
Later changes to the creeds, such as the addition of the filioque in the Nicaean Creed at the regional (non-ecumenical) Council of Toledo, violate the requirement that to be doctrine, the decision of a council must be supported by all of Christendom. Some people see the seven councils as a roadmap to how Protestants, Orthodox, and Catholics can be reunited, healing the schisms in the church.
The term "imperial church" refers to the way Christianity became part of the Byzantine state, a phenomenon sometimes called "caesaropapism."
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