By the early fourth century, the large Christian population of Egypt was making significant cultural and intellectual contributions to the Byzantine world. Coptic, the Egyptian language written with modified Greek letters, was used alongside Greek for liturgical and administrative purposes. Tradition identifies the first patriarch of the Coptic Church as Saint Mark the Evangelist, also the first patriarch of Alexandria. Following the Council of Chalcedon in 451, many in Egypt rejected its decision that Christ had two natures and accepted instead the teaching of Saint Cyril, patriarch of Alexandria (r. 412–44), that Christ's humanity and divinity were united in one nature (Miaphysitism). As a result, in the sixth century there came to be a pro-Chalcedonian ecclesiastical hierarchy in Egypt appointed from Constantinople that sought to suppress the local authority of the Coptic Church. Later, under Arab rule, the legitimacy of the Coptic Church was recognized. Monasticism, always important to Egyptian Christianity, came to shape Coptic liturgy and visual culture. Major monasteries such as those at Bawit and Sohag thrived in the early Islamic era. The church retained Coptic as its primary language until the tenth century, considerably longer than most other non-Muslim communities.