Onesimus is very dear to Philemon on account of the fact that he is a Christian slave and thus someone who will serve him diligently (as a man). Additionally, he is a brother in the Lord, and thus someone who join and serve in the cause of the gospel.1
16 non plus comme un esclave, mais comme supérieur à un esclave, comme un frère bien-aimé, de moi particulièrement, et de toi à plus forte raison, soit dans la chair, soit dans le Seigneur.