Lourenço da Silva de Mendouça

Lourenço da Silva de Mendouça was born around 1650 in the Kingdom of Ndongo in what is now Angola.  Lourenço was the grandson of the King of Pongo, known in our sources as either King Hari or Philipe I. Hari/Philipe ruled from 1626 to 1664 over a breakway state that maintained its independence from Ndongo by allying with the Portuguese. As Portuguese allies, the royal family all of course converted to Catholicism. When Philipe I died, he was succeeded by one of his son, Dom João. According to wikipedia, 

Dom João refused the annual tax of 100 slaves demanded by the colonisers, known as baculamento, and declared war against the Portuguese. 

The Portuguese won the war and decided to send the rebel's family into exile in Brazil. They were not enslaved but lived in rather comfortable circumstances; Mendonça studied in a Catholic seminary. But the Brazilian authorities got suspicious of these African nobles, fearing in particular that they might ally with a free nation of maroons known as the Quilombo dos Palmares. (Maroons who disappeared into the jungle were a major feature of colonial Brazil, and some of their communities endured long enough to have their rights to their land recognized by a left-wing government in the 1990s.) So they split up the family, sending Lourenço and some others to Portugal.

Wikipedia again:

Mendonça was sent to the Convent of Vilar de Frades, in Braga, where he studied for three or four years, before moving to Lisbon.

Notice the importance of noble blood in this world. Mendonça may have been an African, but he was a prince (and a Catholic), so nobody expected him to work for a living. 

Lisbon at that time had a significant black population, estimated at 5 to 10 percent, so the Pumbo royals were hardly alone in their situation. Many of Portugal's Africans joined confraternities, lay Catholic organizations officially devoted to chartity but also functioning as social clubs. In about 1681 Mendonça became the procurator general of the Confraternity of Our Lady, Star of the Negroes, a trans-Atlantic group that also existed in Brazil.

During all of these travels, Mendonça seems to have been working on a legal brief accusing Portugal and Spain of a great crime, the Atlantic slave trade. In 1684 he was able to travel to Rome and present his brief to the papal curia. Mendonça's brief was particularly strong on the gruesome tortures the Portuguese regularly inflicted on rebellious slaves. Mendonça's brief helped to convince Pope Innocent XI to issue a letter reiterating previous Papal condemnations against enslaving Christians and cruelty to slaves. He did not, however, condemn slavery itself.

Mendonça died in Rome in 1698.