The Kingdom of Kongo (1390 - 1914) was a kingdom located in west central Africa in present-day northern Angola, the western portion of the Democratic Republic of theCongo, the Republic of the Congo as well as the southernmost part of Gabon. At its greatest extent it reached from the Atlantic Ocean in the west to the Kwango River in the east, and from the Congo River in the north to the Kwanza River in the south

In 1483, the Portuguese explorer Diogo Cão sailed up the uncharted Congo River, finding Kongo villages and becoming the first European to encounter the Kongo kingdom. Cão left men in Kongo and took Kongo nobles to Portugal. He returned with the Kongo nobles in 1485. At that point the ruling king, Nzinga a Nkuwu, converted to Christianity. King Afonso I created a syncretic version of Christianity that would remain a part of its culture for the rest of the kingdom's independent existence. King Afonso himself studied hard at this task.

 the Kingdom of Kongo became a major source of slaves for Portuguese traders and other European powers.

Edited: https://en.wikipedia.org/wiki/Kingdom_of_Kongo#The_Portuguese_and_Christianity