I always sort of assumed they were all pagan, but recently I found out that between 15 and 30% of them had been Muslims since there were many Muslims in West Africa. It also seems that many were Catholic, but I couldn’t find any info on that except this passage from the Wikipedia article on the Stono Rebellion:

“The slaves were Catholic and some spoke Portuguese, both of which point to Kongo. A lengthy trade relationship with the Portuguese had led to the adoption of Catholicism and learning of the Portuguese language in the kingdom. The leaders of the Kingdom of Kongo had voluntarily converted in 1491, followed by their people; by the 18th century, the religion was a fundamental part of its citizens' identity. The nation had independent relations with Rome.”

So does anyone have any info on what percentages of them followed which religions?