When did the opposition to the slave trade begin?

Answer 1

It is probably as old as slavery itself. Slaves have probably always opposed slavery, and historical records of their efforts to free themselves go back at least to the ancient Greeks. The Biblical story of Exodus implies a strong criticism of slavery. Opposition coming from the group of people who owned slaves is harder to document, because it was not always politically possible to express it, and any written denunciation of slavery stood a good chance of being destroyed as subversive to the slavery system. But we have discussions of the immorality of slavery at least as far back as Latin literature (see Pliny, for example).