The answer is: His slave had run away

v10-12 - it is possible that the slave had stolen from his master (c.f. v18) and therefore run away as under Roman law this was punishable by death.

The short but powerful letter to Philemon showcases a tapestry of relationships woven between Paul, a runaway slave named Onesimus, and his master, Philemon. It opens a fascinating window into the early Christian world, where social structures like slavery were deeply entrenched yet encountering transformative movements towards compassion and equality.

Philemon, a wealthy man in the city of Colossae, likely hosted a church in his home. To think about Onesimus — his enslaved person — being central to this letter curled around like a vine through the walls of this early church transforms the scene from an echo of Roman society into a landscape of radical grace. This letter carved the commerce of human hierarchy and instead proposes a new paradigm of brotherhood.

Interestingly, the name "Onesimus" means “profitable,” adding an ironic twist to his story. While he might have run away and potentially stolen from Philemon, Paul advocates for his return—not with a sense of punitive ownership but rather as a beloved brother in Christ. This highlights early ideas of equality and community that would eventually challenge oppressive structures.

Moreover, the letter exists as one of the few direct addresses about slavery in the New Testament, subtly hinting at a future where the chains of injustice would loosen. It provides not just a historical lens but also a reminder: personal transformation and deeper connections sometimes spring from the most unlikely situations. Thus, it remains a captivating testament to the truth that, even within systems of oppression, there's inexplicable potential for change through sincere relationship and love.