The book of Philemon addresses the story of Onesimus, a slave. What key theological principle does Paul emphasize in his appeal to Philemon regarding Onesimus?

A The importance of maintaining social order and upholding the institution of slavery. B That Onesimus should be severely punished for his past actions. C That Christians should always free their slaves, regardless of economic consequences. D The equality of believers in Christ and the transformative power of forgiveness and reconciliation.