Yes and sort of. Missionary work began almost as soon as Europeans began exploring the African coasts. In addition the Church's official position on slavery was that it was an evil with only one saving grace: it represented an opportunity to turn heathens into Christians and thus save their immortal souls.

When the Spaniards had to start importing slaves to America to replace dwindling numbers of natives, king Ferdinand II began to issue import licenses in 1513 containing various conditions, among them that slaves had to be transported to Spain where they were to be taught Spanish and given a good Christian education before they were sent onwards to the New World. This particular requirement was soon enough abandoned as too onerous. Bozales (i.e. heathens) were found acceptable and it was left up to the church to teach them the true faith in America instead. The licenses did strictly forbid taking Muslim slaves as they might spread false beliefs (something the Spanish crown was very paranoid about). As a consequence traders looked for slaves in Sub-Saharan Africa.

The license system allowed the Spanish crown to earn a fee on each slave without having to invest in establishing a trade network. The lack of official support in turn meant that Portuguese traders were by far best placed to take advantage of the system and came to virtually monopolize the licenses for a very long time. Spanish licenses, first-mover advantage and the labor requirements of their Brazilian colony meant they dominated the transatlantic slave trade until the British eventually surpassed them.

The reason the Portuguese were so well placed was that they had been exploring down the western coast of Africa even before Columbus had found America. Of course missionaries followed in their wake. An early success was the Kingdom of Kongo. The first recorded contact occurred in 1483 and already in 1491 the royal family and leading nobles converted to the Roman-Catholic faith (compare Columbus journey in 1492). However, the king abandoned Christianity in 1495. His son and heir (the future king Afonse I of Congo) remained a Christian and began to push the new faith onto his people, but it was a long process. At first Kongo mainly traded its own slaves but soon enough demand grew so great the kingdom had to resort to raiding neighboring (heathen) people. As such Kongo sold comparatively few Christian slaves to the Portuguese.

Portugal eventually established colonial ports (Luanda, Benguela, Bissau) along the west coast of Africa that broke Kongo's regional monopoly on the slave trade. This also marked a major change in the missionary effort. In the 17th century the Church convinced the Portuguese king that a slave should not be exported unless he had converted to Christianity. A religious official in each Portuguese colony began to issue a certificate as proof of conversion (interestingly he was also the man who determined if a slave was actually a falsely enslaved free Portuguese subject). The church took a fee for each certificate and over time it became the main source of income for colonial parishes.

The whole system was a corrupt joke from the start. The traders couldn't afford major delays because of the great mortality in the slave pens. The church yielded to practical considerations and the desire to save as many souls as possible so minimal time was spent teaching the slaves about Christianity. These lessons were usually done by Africans who spoke the local language. Unfortunately most slaves came from far inland and had no clue what was being said. Add to this that the beliefs of Portuguese-Africans were a synthesis of Christianity and native religions. So even if the teachers could make themselves understood, they would be teaching rank heresy. Ultimately it was a mere formality to get the certificate, except on those occasions when the church was trying to shake down slave traders for more money. So officially an enormous amount of Christians were shipped from Africa, but...