According to James Henley Thornwellwhy were slaves free even though they were still slaves?

James Henley Thornwell, a theologian and spokesman for the pro-slavery movement in the 19th century, argued that slaves were spiritually free because they had the opportunity to embrace Christianity and be saved. He believed that their freedom from sin and the promise of eternal life was more important than their physical bondage. Thornwell's views were part of a theological justification for the institution of slavery in the United States.