Were the Africans captured during the "Trans-Atlantic Slave Trade" already Christians, or did they only become Christians after being captured?

There is a substantial number of people who self-identify as or are otherwise classified as "African-Americans" in the United States who consider themselves Christians. Were the ancestors of those "African-Americans" who were captured, mainly on the west coast of Africa, Christians when they were captured by various African and European war parties, or did those captured Aftricans only become Christians after being captured and embarked to European colonies outside of Africa?