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 letter to Philemon may seem like a brief correspondence focused on a particular slave, Onesimus, but its implications and the culture it reflects make it quite fascinating. Woven into this little epistle is a powerful message of unraveled social norms and personal transformation. Onesimus, who traditionally would have been viewed simply as a fugitive and a piece of property, takes center stage here.

Through Onesimus' escape and subsequent encounter with Paul, we catch a glimpse of a formerly hierarchical worldview on the verge of something radically new. Roman society was elitist, where the majority were slaves and a small number held power and wealth. Yet, Paul does not condone the societal structure but rather challenges it by addressing the inner workings of human relationships.

Interestingly, the idea that community and transformed relationships should come before strict adherence to laws echoes throughout the Bible. A lesson that invites us to envision a world where former enemies could reconcile and share in a renewed kinship.

In a practical sense, when Philemon is urged to receive Onesimus “no longer as a slave, but better than a slave, as a dear brother” (v.16), this astonishing principle pivots on love and understanding—emphasizing dignity over dehumanizing systems.

Through this correspondence, we are reminded of the deep connections and redemption possible amid life’s complexities, reinforcing themes prevalent in many other biblical writings, invoking ideas of freedom and mutual respect.