The Byzantine Empire was the remaining land of the Eastern Roman Empire that fell in 450 AD. Constantine I rebuilt the empire, reintroduced Christianity and moved the capital to Constantinople. The Byzantine Empire had many enemies who included the Persians, European barbarians, and central Asian nomads.
With the help of the Christian Church, they solidified and solved their problems as the Christian Chruch had a system of tithes and a sense of religious unity that the government lacked on. On the other side, Islam began in Mecca and Medina in 610 A.D. after Mohammed experienced an angelic visitation.
Muhammad made it his life mission to spread and teach what was dictated on the Qur’an. After his death, the Muslim community had no leader as Muhammed died without naming his successor. This is when caliphs were born, which were the leader of caliphates. Caliphates grew rapidly through conquest and used religion to further expand and govern (Jones).