The Byzantine Empire was the eastern half of the remaining Roman Empire. Constantine I took over the Roman Empire after winning the he Battle of the Milvian Bridge. During Constantine’s rule, he rebuilt the empire, reintroduced Christianity and moved the capital to Constantinople.
This laid the foundations of the Byzantine Empire. This empire relied a lot on the Christian Church. With the help of the Christian Church, they solidified and solved their problems. Through a system of tithes, the Christian Church received a lot of money and their sense of religious unity helped the government increase its power over the people. On the other side, Islam began in Mecca in 610 A.D. after Mohammed experienced an angelic visitation from Angel Gabriel.
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 he left no successor. This is when caliphs were born, which became the leaders of caliphates. Caliphates grew rapidly through conquest and used religion to further expand and govern.