January 2 1596. Agustino Nani, Venetian Ambassador in Spain, reports to the Doge and Senate on the arrival of the Congolese ambassadors. Their entreaty was addressed to Philip II of Spain and Portugal. At first sight it seems somewhat brazen and distinctly unchristian, in that it would effectively allow Portugal to trade in non-Christian negro slaves, leaving Christian converts free. It was, however, underpinned by pressing if unpalatable economic realities. I add below an extract from a SAHO article on slavery in the Congo together with a link to the full text.
Two Ambassadors, uncle and nephew, have arrived here from a King of the Congo provinces, to renew their obligations towards his Majesty, as King of Portugal, and to beg him to prohibit the Portuguese in the region of Cape Verd and along that whole coast, from buying the negroes who are Christians. Some of the Portuguese, stimulated by the ease with which these negroes could be sold as slaves owing to their natural qualities, have been making large purchases; and this has led the neighbouring princes to capture these negroes in order to sell them again.
Furthermore, for their better education in the Catholic Faith, the Ambassadors desire leave to build a cathedral church and demand the appointment of a bishop of their own, for at present in their spiritual affairs they are compelled to go to the Bishop of St. Thomas.
For a full account of slavery in the Congo see SAHO. Here is a relevant extract from their article.
A major obstacle for the Kingdom of Kongo was that slaves were the only commodity which foreign powers were willing to trade for, and this meant that Kongolese kings had no international currency other than people. Slaves became the tool through which Kongo developed and sustained their material, cultural and diplomatic ties with the European powers. Kongolese nobles could buy slaves with the local currency, nzimbu shells, and the slaves could in turn be traded for international currency. As an example of how slaves were used as an international currency we can see how the Kongolese authorities paid the Catholic church in slaves for bishops to preform various religious duties in the Kingdom.