What does Philemon 1:12 mean?

Verse 12 continues Paul's description of his relationship to Onesimus. First, he says that he is sending Onesimus back to Philemon. This was a major risk for Onesimus, since a runaway slave could be punished with death. Therefore, Paul wanted to emphasize that the return was something he had arranged personally.

Second, Paul indicates that he and Onesimus had become very close in a short amount of time. He describes Onesimus as a part of himself, as his "very heart." Paul had led him to faith in Christ, becoming his spiritual father (Philemon 1:10). Onesimus had been so helpful to Paul that he wanted to keep him in Rome to assist him while imprisoned (Philemon 1:13).

Philemon had refreshed the hearts of the saints (Philemon 1:7), and Paul would later ask him to "refresh my heart in Christ" (Philemon 1:20). Paul uses the same term for "heart" to express his great love for Onesimus. Again, this is Paul pleading with Philemon to choose forgiveness for the runaway slave, rather than having it commanded to him.