What is the theological significance of Onesimus' transformation from a runaway slave to a 'useful' brother in Christ, as portrayed in Paul's letter to Philemon?

A It demonstrates the economic benefits of Christian conversion for slaves. B It highlights the superiority of Christian slaves over pagan slave owners. C It illustrates the power of forgiveness and reconciliation in overcoming social barriers and the transformative nature of the Gospel. D It serves as a justification for the continuation of slavery within a Christian context.