The book of Philemon addresses a specific situation involving a slave named Onesimus. What is the central theme or purpose of Paul's letter to Philemon regarding Onesimus?

A To demand Philemon punish Onesimus severely as an example to other slaves. B To instruct Philemon to sell Onesimus for a higher price and donate the proceeds to the church. C To encourage Philemon to receive Onesimus back not merely as a slave, but as a beloved brother in Christ, forgiving him for any wrongdoing. D To publicly denounce the institution of slavery and call for a complete social revolution.