v1-3: Paul's opening greeting. The letter was written from prison in Rome, at the same time as the letters to the Ephesians and the Colossians. Tychicus and Onesimus brought the letters from Rome, see Eph 6.21 and Col 4.7-9.
Philemon was a dear friend and fellow worker, see Prov 27.10; Rom 12.10. Apphia and Archippus were similarly beloved. There was clearly a strong friendship between Paul and these people. They served God in different places, but enjoyed a unity in Christ. We may compare their friendship with that between David and Hushai (2 Sam 15.32-37; 16.15-17.14).
The church met in Philemon's house; see Rom 16.3-5; 1 Cor 16.18-19; 2 Cor 7.13. This was a common N.T. practice.
v4: Paul constantly prayed for Philemon, and with gratitude.
v5: Paul had heard continuing good news about Philemon's character. Mentioned here are his faith in the Lord Jesus, and his love for all the saints.
v6: Paul encouraged his witness and testimony; we do need to share our faith. This has the benefit that we grow in understanding in the faith.
v7: Paul's testimony from others, that Philemon had brought blessing to many. He had refreshed their hearts.
v8-9: The letter was written out of love; it is a personal letter, not dealing with church order or doctrine. Paul was motivated by love and compassion, and a desire that God be honoured in the lives of Philemon and Onesimus.
Paul writes as "an old man" (v9), not an apostle. Paul laid aside his authority; he did not command Philemon, he appealed (Gk: PARAKALEO, to call to one's aid); this reflects a relationship and affection, rather than a legal or moral obligation.
We are reminded also that Paul was in prison, and therefore limited in what he could do. In this situation, he could only write, although he was confident of Philemon's response as a godly man.
v10: "My son Onesimus", since Onesimus had been converted to Christ through Paul. Having fled from Philemon, he had found his way to Rome and to Paul. We note God's power in bringing him to Paul.
v11: Conversion changes people; the unprofitable becomes profitable. The thief must work and be generous, Eph 4.28. There is a pun here, as "Onesimus" means 'profitable.' He was now living up to his name. See also the description in Col 4.9.
v12: Paul was sending Onesimus back to Philemon. This was his responsibility, since Onesimus was Philemon's servant. Onesimus himself had a duty to return.
"My very heart", Onesimus had become dear to Paul, and doubtless Paul would have enjoyed further fellowship with him.
v13: Paul wanted to keep Onesimus, thinking of him as Philemon's representative. Paul was perhaps hoping that Philemon might release him to serve with Paul.
v14: But Paul would not keep Onesimus without Philemon's permission and approval.
v15-16: The new relationship between Philemon and Onesimus. The wording here is similar to that in Is 54.8, describing God's eternal kindness upon His people Israel. Philemon should take Onesimus back as a brother was a massively subversive idea, it would be unheard of in that day.
In Ex 21, the Hebrew slave could choose to stay with his master forever. In Christ, similarly, there is a motivation of love, "as a dear brother."
In 1 Tim 6.2, the believing slave must not despise the believing master, since the legal authority still remained. The slave should serve him even better.
v17: "Welcome him" (or 'receive him'); the responsibility of all of us is to be forgiving, see Rom 15.7. Christ has received us, with all our wrongs; we must learn to receive others, holding no offence nor grudge.
Welcome (Gk: PROSLAMBANO) infers a real welcome; used in Acts 28.2; Rom 14.1, 3; 15.7.
v18: Onesimus, a fleeing slave, had probably stolen from Philemon. Paul did not excuse or cover such wrong. But he did offer to make full restitution; "charge it to me" ('put that on my account").
Christ has paid for our wrongs, our offence against God; He has put it on His own account. The just has suffered for the unjust (1 Pet 3.18), that the relationship might be restored.
Christ has accounted for our wrongs, therefore we are received, and we can receive others, for He has paid for their wrongs also. Out of love, we may be constrained to pay for the wrongs of others.
v19: "I will pay it back", Paul's repeated offer. This answers any question in Philemon's mind. Paul reminded him that he had been converted to Christ through Paul also.
v20: Paul had a deep affection for Philemon, his brother; not only had the hearts of the saints been refreshed, but Paul's heart had also. See also 3 John 3, 12.
v21: Paul was confidence of his obedience, that he would do more than was requested. Our lives must be characterised by obedience, so that others might know that we will always obey.
v22: The effectiveness of prayers. Paul looked forward to renewing fellowship with Philemon, and even asked that a room be prepared for his arrival.
v23-25: Closing greetings and blessing.
There is a postscript to the letter; if Onesiphorus is Onesimus, then 2 Tim 4.19 refers to a household; the returned slave had a family, and was part of the church.
We might ask why Paul did not recommend the abolition of slavery, in the way that William Wilberforce and other did in the 19th century. Slavery was certainly not always accompanied by mistreatment, and this letter is evidence of that. ‘to overthrow the institution of slavery would have ruined the economic foundations of every major civilisation before the Industrial Revolution.’ (C R Dickinson, letter to The Times 27 Nov 2010)