The purpose of this paper is to argue that religious culture was the main cause for differences in how Arabs and Portuguese treated East Africans. The history of Portuguese and Arab interaction with East Africans will be examined to show the way the two imperialistic powers treated East Africans. Examples of treatment that differ from the norm will be examined to show that there are not many examples of treatment other than what is found in East Africa. This paper will examine the Qur’an and the Bible to explain the way that each religion instructs its followers to treat people of different religions. This paper will also examine teachings of the leaders of Islam and Christianity at the times that the two groups arrived in East Africa. The historic and religious examples will then be drawn together to further argue that religious culture was the main cause for differences in how Portuguese and Arabs treated East Africans. The cultures of Arabs and Portuguese will be examined to show that the two groups had a lot in common. Both were imperial, trading nations with significant economic and militaristic power. Examining the religions of the two groups and the cultures created by religious leaders and writings will show that religious culture was the most significant difference between the two groups and therefore the cause for the different treatment of East Africans.