The occasion for Paul writing to Philemon is because another Christian brother, Onesimus, is with Paul. The trouble is that Onesimus is a runaway slave, belonging to Philemon. To the best of our knowledge, he ran away to accompany the apostle who is being led in captivity to Rome where his appeal to the emperor will fail and he will have his head cut-off, thus failing to realize his intention of moving westward from Rome to spread the Gospel. It is clear that Paul loves Onesimus, who would not love another willing to accompany you, at such great risk to himself, under adverse circumstances? This is why the apostle states that he would like "to retain him for myself, so that he might serve me on your behalf in my imprisonment for the gospel" (Phil v. 13). It is clear that it is difficult for Paul to let Onesimus go, but, Paul states that he does not want "to do anything without [Philemon's] consent, so that the good [he does] might not be forced but voluntary" (v. 14).
I attended a vigil Mass last night with my two oldest children. I was struck and deeply moved upon hearing these words from Paul to Philemon, referring to Onesimus: "I am sending him, that is, my own heart, back to you" (v. 12). This, I thought, is true companionship! I can't imagine that Onesimus, despite likely being very disappointed and even a little heart-broken at being sent back, would not go at the behest of St. Paul. We do not know the rest of the story, did Onesimus go? Did Philemon receive him back with brotherly love? Assuming Onesimus did go, it is easy to imagine the parting between the apostle and his devoted companion, who came to be with him at such great risk to himself, a parting that was made in all probability with the realization that they would never see each other again. Yet, it was not hopeless because they clearly both trusted in the One who brought them together in the first place.