A group of Christian churches that do not recognize the authority of the pope in Rome, but, like the Roman Catholic Church, have roots in the earliest days of Christianity. The Eastern Orthodox churches split from the Western church in the Great Schism of 1054, primarily over papal authority and whether the Holy Spirit proceeds from the Father (as the Orthodox believe) or from the Father and Son (as the Catholics believe). Included in the Eastern Orthodox churches are the Greek Orthodox, Russian Orthodox and Armenian Orthodox, as well as other, smaller churches based on the nationalities of various ethnic groups such as Bulgarians, Romanians and Syrians. Eastern Orthodox clergy comparable to Catholic archbishops are known as patriarchs or metropolitans. They recognize the patriarch of Constantinople, now Istanbul, as their leader. He has the power to convene councils, but he does not have authority over the activities of the other archbishops. The patriarch of Constantinople is known as the ecumenical patriarch. Working with the archbishop are other archbishops, bishops, priests and deacons. Archbishops and bishops frequently follow a monastic tradition in which they are known only by a first name. When no last name is used, repeat the title before the name in subsequent references. Archbishop may be replaced by the Most Rev. on first reference. Use the Rev. before the name of a priest on first reference. On second reference use only the cleric’s last name. The churches have their own traditions on matters such as married clergy; for example, a married man may be ordained, but a priest may not marry after ordination. In the United States, the largest Eastern Orthodox church is the Greek Orthodox Archdiocese of America, followed by the Orthodox Church in America.