In the book of Philemon, what theological principle does Paul subtly emphasize through his appeal on behalf of Onesimus, a runaway slave?

A The continued acceptance of slavery as a social institution, provided slaves are treated kindly. B The social hierarchy of the Roman Empire should be maintained, regardless of one's faith. C The transformative power of Christ to erase social barriers and foster reconciliation between individuals, regardless of status. D Philemon is obligated to free Onesimus only if Onesimus repays him for his lost services.