Considering the historical context of the Roman Empire and its societal structures, what is the subversive nature of Paul's letter to Philemon regarding the slave Onesimus?

A It reinforces the existing social hierarchy by subtly reminding Onesimus of his duties as a slave. B It primarily aims to secure financial support for Paul's missionary journeys by leveraging Philemon's wealth. C It undermines the Roman social order by advocating for the spiritual equality of slaves and masters in Christ, implicitly challenging the institution of slavery through reconciliation and brotherhood. D It solely focuses on personal forgiveness between Philemon and Onesimus, disregarding the broader implications of slavery within the Roman Empire.