It’s a good thing you put “Christian” in quotes because to some there is no difference between cultural Christians and actual Christians. There is in fact a difference, but this is the very basis on how some claim that Hitler was also a Christian, although never calling himself one. Western culture is basically what’s left of what was once known as “Christendom”, having now fallen somewhat from that.
Unfortunately, most of those who engaged in the slave trade were actual Christians, to use your terms, at least in their own minds and those of their contemporaries.
I used the quotes because IMO slavery is in conflict with the basic ideas and concepts of Christianity. Unfortunately, the actual New Testament contains no specific denunciation of the institution, with the writers apparently accepting it as just a fact of life.
As indeed apparently all human societies did until Christians began a campaign against it in the 18th century.