Contrary to other African peoples the Kongo did not have a strong mask tradition. They were worn by the nganga diphomba (ritual specialist or diviner). The term ngobudi, that refers to them, means something frightening, a spirit or power that induces terror. Once upon a time, the Kongo ethnic group formed a prosperous kingdom, which declined with the arrival of the Portuguese in 1500; The Portuguese attempted to Christianize the Kongo people, overpowering their spiritual practices and folk beliefs. The art Kongo has favored issues concerning the worship of ancestors, of the earth, of the sky and of the supreme god Nzambi.