Answer:
difference The Byzantine Empire's legal system was based off their predecessors, the Romans. The Islamic Caliphates' legal system was based off of the Qumran and Hadith, named Sharia Law.
Similarity The Byzantine Empire and Islamic Caliphates appointed their political leaders as religious leaders also. They did this, because they would have more power over the people; not only did they regulate their laws and duties, but also their religion. They would gain favor from the public for embracing religion.
The Byzantine Empire and Islamic Caliphates both used religion as a way to unite their people, and as a justification for expansion. Islam began small in Mecca, and the Byzantine was just a fraction of the Roman Empire - but through the use of religion they expanded. The Islamic Caliphates used the idea of the Jihad, while the Byzantine Empire under Justinian similarly used Christian teachings, and the idea of spreading the faith to other areas - to gain control of the land that the Western Roman Empire lost.
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