Middle Ages
Church life

The idea of Christian unity endured, although differences in ideology and practice between the Eastern and Western Churches were increasing. Native Roman aversion to Arian conquerors reinforced the traditional Christian concept of the separation of church and state in the west; this concept was alien to eastern clergymen, however, who regarded the Roman state as an instrument of divine providence. After the Muslim conquests, Byzantine emperors could less effectively intervene in the west. When Leo III (r. 717–41) prohibited the display of paintings of human figures in places of worship, the papacy rejected his claim to declare new dogmas with imperial edicts. Although the Byzantine Church condemned iconoclasm in 843, issues such as the rivalry for ecclesiastic jurisdiction over newly-converted peoples and the unilateral modification of the Nicene Creed in the west widened to the extent that differences were greater than similarities. In the west, the tithe (originally a voluntarily contribution) began to be levied as a church tax on agrarian products during the 10th century.

Few western bishops looked to the papacy for leadership. The only part of Western Europe where the papacy had influence was Britain, where Gregory the Great (pope 590–604) had sent a mission to convert the Anglo-Saxons to Christianity. Church attendance was low, and meetings with itinerant clergy and pilgrimages to popular saints' shrines were instrumental in religious education. Irish missionaries were most active in Western Europe between the 5th and 7th centuries. They were the first to use handbooks, known as penitentials, to determine appropriate acts of penance —typically prayers and fasts —for sinners. They emphasised sexual morality and prescribed severe penances for adulterers, fornicators and those engaged in non-reproductive sexual acts (such as homosexuals). In contrast with official Christianity, the Bogomils in the Balkans condemned sexual reproduction because they considered Satan the creator of the physical universe.

The Early Middle Ages saw the rise of Christian monasticism. Monastic ideals spread from Egypt in hagiographical literature, especially the Life of Anthony. Most European monasteries focused on community experience of the spiritual life, known as cenobitism. The Italian monk Benedict of Nursia (d. 547) developed the Benedictine Rule, which was widely used in western monasteries. In the east, monastic rules compiled by Theodore the Studite (d. 826) gained popularity after they were adopted in the Great Lavra on Mount Athos during the 960s.

Monasteries had a deep effect on local society, acting as land trusts for powerful families and centres of political authority; they were the main (and sometimes only) outposts of education and literacy in a region. Many surviving manuscripts of the Latin classics were copied by monks. Monks were also the authors of new works on history, theology, and other subjects by authors such as Bede (d. 735), a native of northern England. The Byzantine missionary Constantine (d. 869) developed Old Church Slavonic as a new liturgical language, establishing the basis for a flourishing Slavic religious literature; a new script was adopted c. 900, now known for Constantine's monastic name as Cyrillic. The Saxon nun Hrosvitha (d. 1000) wrote the first non-liturgical medieval dramas.

In Western Christendom, lay influence over church affairs reached its apex during the 10th century. Aristocrats regarded the churches and monasteries under their patronage as their personal property, and simony —the sale of church offices—was a common practice. Simony aroused a general fear, since many believed that irregularly-appointed priests could not confer valid sacraments such as baptism. Monastic communities were the first to react to this fear with rigorous observance of their rules. The establishment of Cluny Abbey in Burgundy in 909 initiated a more radical change, since Cluny was freed from lay control and placed under the protection of the papacy. The Cluniac Reforms indicated that the reformist idea of the " Liberty of the Church " could be achieved with submission to the papacy.
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