The Western (Latin) and Eastern (Greek) divisions of Christianity began to take on distinctive shape in 7th-century Christianity. Whereas in the East the Church maintained its structure and character and evolved more slowly, in the West the Bishops of Rome (the popes) were forced to adapt more quickly and flexibly to drastically changing circumstances. In particular, whereas the bishops of the East maintained clear allegiance to the Eastern Roman emperor, the Bishop of Rome, while maintaining nominal allegiance to the Eastern emperor, was forced to negotiate delicate balances with the "barbarian rulers" of the former Western provinces. Although the greater number of Christians remained in the East, the developments in the West would set the stage for major developments in the Christian world during the later Middle Ages.
During the 7th century an Arabian religious leader named Muhammad ibn ‘Abdullāh began to spread the message of the Qur'an (Koran), which includes some traditions similar to those of the Christian and Jewish faith. This new faith, called submission or الإسلام (al-’islām) in Arabic, proclaimed the worship and obedience of a purely monotheist God or Allah in Arabic as the purpose of life, and Islam would ultimately prove to be the greatest challenge that the Christian Church would face during the Middle Ages. By the 630s Muhammad had united the entire Arabian peninsula under Islam, including the formerly Christian kingdom of Yemen. Following Muhammad's death a Muslim empire, or caliphate, emerged which began efforts to expand beyond Arabia. Shortly before Mohammad's death the Roman Empire and Sassanid Persian Empire had concluded decades of war, leaving both empires crippled.