The book of Philemon addresses a specific situation regarding Onesimus. What is the central theme or message that the Apostle Paul is conveying through this personal letter?

A A call for the complete abolition of slavery within the Roman Empire. B An encouragement for Philemon to financially support Paul's missionary journeys. C A demonstration of how to properly conduct business dealings according to Christian principles. D A plea for Philemon to forgive and receive Onesimus, not as a slave, but as a beloved brother in Christ.