The answer is: He had become a brother in Christ

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In Paul's letter to Philemon, we see a profound transformation take place not just in the man's social status but in his very identity—Onesimus, this once-fugitive slave, has embraced faith and grown spiritually! By referring to him as “a brother in Christ,” Paul isn’t just giving a warm greeting; he’s showcasing a radical shift in social norms. In the ancient world, the relationship between slave and master was defined strictly by socioeconomic status, but in Christ, these barriers were bridged.

During the early days of the church, many converts were initially from diverse backgrounds, ranging from the wealthy to the socially marginalized. This emphasis on brotherhood reflects a burgeoning insight of equality and human dignity, which would someday influence broader societal transformations—and remain a relevant conversation today.

Paul’s personal touch is also notable. His affection for Onesimus turns what could be an ordinary clerical correspondence into a heartwarming narrative about forgiveness and reconciliation. Interestedly, the name “Onesimus” means “useful,” aligning beautifully with Paul’s irony of stating, "He was once unprofitable to you," now becoming useful at multiple levels.

Moreover, the work of reconciliation branded within this concise letter has sparked extensive theological discussions through time and continues encouraging genuine dialogues about relationships, reform, and equal regard for individuals regardless of their backgrounds. Isn’t it intriguing how a simple letter could challenge centuries of values?