ISB ~ Philemon 1 v16

no longer as a slave:Philemon is faced with a dilemma. As a slave owner, he is entitled by law to punish the returning Onesimus to the fullest extent (death). As a Christian, however, he must acknowledge that the recent conversion of Onesimus has put him and his slave on an equal footing in the eyes of God (Gal 3:28). In Paul’s mind, there is only one recommended option: Philemon must embrace Onesi-mus as his brother in the faith, forgive him his wrongdoing, and give him his freedom. Christ has made them brothers, and this creates a new situation that overrides the social and legal expectations that would normally apply when a delinquent slave returned to his owner. These men were once members of the same household, with one in authority over the other; but now they are children of equal standing in the household of God the Father (Philem 3). This is one example of how Paul, who never condemned the institution of slavery directly, worked against it with the gospel (CCC 2414). See note on 1 Cor 7:21.

• Sometimes evil circumstances become an occasion for good, and God turns the evil plans of men toward a righteous end. If Onesimus had not fled his master, he would not have come to Paul in prison and there received faith in Christ (St. Jerome, Commentary on Philemon on Philem 15).