This detail suggests that Olaudah Equiano's audience were the Europeans who indulged in slave trade. He asks this question to appeal to their conscience based on their Christian beliefs.
Olaudah Equiano was a notable African of Igbo descent who was sold from Africa's West Coast to the Caribbeans and later to London. He wrote several accounts of his experience and emotions as a slave as well as the ill-treatment meted on him and his fellow slaves by the Europeans. After enumerating the several hardships faced by the African slaves, he asks the question above.
From the wordings of the question, a reader can infer that it was directed to his captors. They believed in Christianity and the Bible but were apparently not heeding to the requirements of their God who instructs that they do to others just as they would want to be treated.