In the book of Philemon, what is the underlying theological significance of Paul's urging Philemon to receive Onesimus back, not just as a slave, but as a brother in Christ?

A It was primarily a request for Philemon to manumit Onesimus, setting a precedent for the abolition of slavery. B It highlighted the transformative power of the Gospel to break down social barriers and establish equality among believers, challenging the hierarchical structures of the time. C It showed Paul's willingness to interfere in Philemon's personal affairs. D It was a subtle way for Paul to assert his authority over Philemon.