What does Philemon 1:15 mean?

Verse 15 continues Paul's plea to Philemon to forgive Onesimus and set him free. It also adds a possible explanation behind Onesimus' escape. Paul chooses to say that Onesimus "separated," or "parted," from Philemon, rather than saying he "ran away." This was probably to soften the fact that Philemon was a runaway slave subject to legal punishment. These words reflect Paul's teaching that God has a positive purpose for everything that happens in the life of the believer (Romans 8:28). Though Paul does not say so with certainty, he suggests the bad event that had happened (Onesimus' running away) has led to a positive outcome (his return as a believer).

The comment that Philemon could get Onesimus "back forever" has been interpreted in two ways. First, it could be taken to mean Onesimus would return to Philemon as a slave, and remain so for the rest of his life. However, another interpretation is more likely: that Onesimus would return as a brother in Christ. This was made clear already (Philemon 1:10) and will be stated again in verse 16. As a fellow Christian, Philemon and Onesimus would be together forever, both in this life and in the next.