The religious history of the Kongo is complex, particularly after the ruling class of the Kingdom of Kongo accepted Christianity at the start of the 16th century. According to historian John K. Thornton "Central Africans have probably never agreed among themselves as to what their cosmology is in detail, a product of what I called the process of continuous revelation and precarious priesthood." The Kongo people had diverse views, with traditional religious ideas best developed in the small northern Kikongo-speaking area, and this region neither converted to Christianity nor participated in slave trade until the 19th century.