How did the Kingdom of Kongo become Christian and lose its people to the slave trade?

How did a Central African kingdom become Christian, and lose its people to the Atlantic slave trade? Both stories began with the same Portuguese ships.

In 1483 the Portuguese explorer Diogo Cão reached the coast of Kongo and took Kongo nobles to Portugal. Within a few years the ruling king, Nzinga a Nkuwu, decided he would become Christian. He sent a large mission to Europe, where its members stayed nearly four years studying Christianity and learning to read and write. In 1491 priests baptised the king, who took the name João I. Afonso I, who took the throne in 1509, went further: he created a syncretic version of Christianity that remained part of the kingdom's culture for as long as it was independent. A Portuguese chaplain admitted that Afonso knew more about church teaching than he did. Afonso's son Henrique was sent to Europe to be educated and was named a bishop in 1518.

But the same contact fuelled the slave trade. Kongo had slavery from its beginning, with war captives taken to the capital, and that same system made it an efficient exporter. After Portugal's colony of São Tomé developed sugar, Kongo became a major source of slaves for it. The trade gradually eroded the king's power.

Afonso saw this and protested. He wrote to the king of Portugal that his people were being kidnapped each day and that his land was entirely depopulated. In 1526 he threatened to stop the slave trade altogether. Yet Afonso had himself taken part: by the 1510s he had accepted Portuguese military help in exchange for a share of the captives from Kongo's wars.