The official religion of the Byzantine Empire was Christianity, as defined by the patriarch and the church hierarchy in Constantinople and by the emperor. In the sixth and seventh centuries, efforts to enforce loyalty to the Orthodox faith met with resistance from Christian communities in the empire's southern provinces. Central to these debates was the acceptance or the rejection of the official definition of the person of Christ as having two natures—human and divine—as formulated at the Council of Chalcedon in 451. The efforts of Emperor Heraclius (r. 610–41) to promote a compromise only intensified the debates. With the shift of authority to Islamic rule, Christians loyal to Constantinople's church generally remained in the eastern Mediterranean, as far south as the Sinai Peninsula. Sites such as the patriarchate of Jerusalem and the monasteries of Mar Saba near Jerusalem and Saint Catherine on the Sinai Peninsula remained important intellectual centers for these communities, whose people wrote in Greek, Christian-Palestinian Aramaic, and ultimately Arabic. Because of their loyalty to the Orthodox Church in Constantinople, these Christians were known as Melkites ("royalists"). During the first centuries of Islamic rule, Orthodox and other Christians held important posts in the Islamic administration and played a vital role in the intellectual life of the major cities.