Eastern Orthodox Christian theology - Wikipedia
A brief overview of the early history of the Orthodox Church
Charlemagne’s crowning made the Byzantine Emperor redundant, and relations between the East and the West deteriorated until a formal split occurred in 1054. The Eastern Church became the Greek Orthodox Church by severing all ties with Rome and the Roman Catholic Church — from the pope to the Holy Roman Emperor on down.
Christianity A brief overview of the Orthodox Church's early history
Eastern Orthodox Catholics and Roman Catholics are the result of what is known as the East-West Schism (or Great Schism) of 1054, when medieval Christianity split into two branches.