The answer is: He had become a brother in Christ

v16

In the brief but impactful letter to Philemon, the transformation of Onesimus, a runaway slave, reflects deep theological and social implications of early Christianity. While he started as a slave, Paul reveals that during his time away, Onesimus has embraced the faith, becoming "a brother in Christ" (v16). This shift isn’t merely personal; it signifies a radical redefinition of social relationships under Christ’s teachings.

Historically, Paul’s inclusive vision was significant during a time when social hierarchies dominated society. The radical idea that slaves could be recognized as equals within a spiritual community challenged longstanding norms. It’s like previewing a family reunion where everyone gets along and shares love equally, regardless of their past roles or statuses. This echoes the teachings found in Galatians 3:28, where Paul states there is neither Jew nor Gentile, slave nor free, male nor female, for all are one in Christ Jesus.

Interestingly, Onesimus's name means "useful," which carries additional weight in understanding his transformative journey – from being regarded as a mere piece of property to embodying true usefulness in the fellowship of believers. His story illustrates the power of redemption and how one’s identity can evolve in the light of faith. Scholars speculate that Onesimus later became a leader in the early church, emphasizing that no matter one’s past, a radical shift toward compassion and brotherhood can create lasting community ties.