Russian Orthodoxy (Russian: ? ) is the body of several Churches within the larger communion of Eastern Orthodox Christianity, whose liturgy is or was traditionally conducted in Church Slavonic language. Most Churches of the Russian Orthodox tradition are part of the Eastern Orthodox Church.
Historically, the term "Greek Orthodox" has been used to describe all Eastern Orthodox churches, since the term "Greek" can refer to the heritage of the Byzantine Empire. However, after the fall of Constantinople, the Greek influence decreased. Having lost its Christian basileus after the Turkish conquest, Constantinople, as a center of power, lost a significant part of its authority. On the other hand, the Moscow rulers soon began to consider themselves real Tsars (this title was already used by Ivan III), and therefore, according to them, the center of the Eastern Orthodox Church should be located in Moscow, and thus the bishop of Moscow should become the head of Orthodoxy. With some Eastern Orthodox people calling Moscow the "Third Rome", or the "New Rome", the Russian Church gained influence in the orthodox world outside the Ottoman Empire. After this event, a series of doctrinal and liturgical differences would emerge in the Slavic Orthodox world, being cut off from its Greek counterpart. By the mid 17th century the religious practices of the Russian Orthodox Church were distinct from those of the Greek Orthodox Church. Eventually, Patriarch Nikon of Moscow would reform the Church and bring most of its practices back into accommodation with the contemporary forms of Greek Orthodox worship. This change, however, was rejected by a large group of traditionalists, who would come to be known as Old Ritualists.