Well, they could have known just like they can today: they could see that these people were suffering. Indeed, there were some people, or should I say organizations, in the United States who would buy slaves or obtain them in different ways and then free them and send them back to Africa.
teachings of the Bible are so muddled and self-contradictory that it was possible for Christians to happily burn heretics alive for five long centuries. It was even possible for the most venerated patriarchs of the Church, like St. Augustine and St. Thomas Aquinas, to conclude that heretics should be tortured (Augustine) or killed outright (Aquinas). Martin Luther and John Calvin advocated the wholesale murder of heretics, apostates, Jews, and witches. - Sam Harris, "Letter To A Christian Nation"
