Justinian I. Of these five, one was in the West (Rome) and the rest in the East (Constantinople, Jerusalem, Antioch, and Alexandria). Even after the split of the Roman Empire the Church remained a relatively united institution (apart from Oriental Orthodoxy and some other groups which separated from the rest of the state-sanctioned Church earlier). The Church came to be a central and defining institution of the Empire, especially in the East or Byzantine Empire, where Constantinople came to be seen as the center of the Christian world, owing in great part to its economic and political power. Once the Western Empire fell to Germanic incursions in the 5th century, the (Roman) Church became for centuries the primary link to Roman civilization for medieval Western Europe and an important channel of influence in the West for the Eastern Roman, or Byzantine, emperors. While, in the West, the so-called orthodox Church competed against the Arian Christian and pagan faiths of the Germanic rulers and spread outside what had been the Empire to Ireland, Germany, Scandinavia, and the western Slavs, in the East Christianity spread to the Slavs in what is now Russia, south-central and eastern Europe. The reign of Charlemagne in Western Europe is particularly noted for bringing the last major Western Arian tribes into communion with Rome, in part through conquest and forced conversion. Starting in the 7th century, the Islamic Caliphates rose and gradually began to conquer larger and larger areas of the Christian world. Excepting North Africa and most of Spain, northern and western Europe escaped largely unscathed by Islamic expansion, in great part because richer Constantinople and its empire acted as a magnet for the onslaught. The challenge presented by the Muslims would help to solidify the religious identity of eastern Christians even as it gradually weakened the Eastern Empire. Even in the Muslim World, the Church survived (e.g., the modern Copts, Maronites, and others) albeit at times with great difficulty. Great Schism of 1054 Although there had long been frictions between the Bishop of Rome (i.e., the patriarch of the Catholic Church proper) and the eastern patriarchs within the Byzantine Empire, Rome's changing allegiance from Constantinople to the Frankish king Charlemagne set the Church on a course towards separation. The political and theological divisions would grow until Rome and the East excommunicated each other in the 11th century, ultimately leading to the division of the Church