Onesimus, what does the Bible say about him? Not a lot. The name occurs twice in the scripture. Once in this Sunday’s Reading:
Philemon 1: 9 to 17:
 Yet for love’s sake I rather beseech thee, being such an one as Paul the aged, and now also a prisoner of Jesus Christ.  I beseech thee for my son Onesimus, whom I have begotten in my bonds:  Which in time past was to thee unprofitable, but now profitable to thee and to me:  Whom I have sent again: thou therefore receive him, that is, mine own bowels:
 Whom I would have retained with me, that in thy stead he might have ministered unto me in the bonds of the gospel:  But without thy mind would I do nothing; that thy benefit should not be as it were of necessity, but willingly.  For perhaps he therefore departed for a season, that thou shouldest receive him for ever;  Not now as a servant, but above a servant, a brother beloved, specially to me, but how much more unto thee, both in the flesh, and in the Lord?  If thou count me therefore a partner, receive him as myself.
was he Paul’s son? He calls him ‘mine own bowels’! Other translations render, ‘mine own heart.’ The word has also been translated as ‘affection’ in other verses. Now there are two verses in Philemon in which this translation fits, verses 7 & 20.
Also Col. 4: 9 mentions an Onesimus, who may be the same person.
In 1. Tim. 1: 2 Paul called Timothy ‘[my] own (other translations: true) son in faith.’ The same phrase Paul used to address Titus in Tit. 1: 4. The word gnesios is strongly related to birth, genesis and the begetting of children. Nevertheless the phrase ‘own (or true, gnesios) son on faith’ seems to indicate that these two are not his own biological children.
Reading the Letter to Philemon
it appears that Onesimus was a slave. Apparently he has fled from Philemon and Paul sent him back.
Philemon 1: 10, says that Paul had ‘begotten him in my bonds.’ Some translations render imprisonment instead of bonds. The interpretation is that Paul had converted Onesimus while being in prison in Rome.
However, it seems quite unlikely that a runaway slave would make his way from Colossae in today’s Turkey to Rome. And why would he seek to find Paul before his conversion? So maybe Paul had written this letter at an earlier date. And he had sent the letter together with his biological son back to Philemon. The reference in Philemon 1: 13, that Paul would have liked Onesimus to serve him ‘in the bonds of the gospel,’ is not necessarily a reference to prison. It may very well be a reference to his service to God without any thought of himself. A service in bonds!
In the end of the letter (Philemon 1: 22) Paul wrote that he anticipated to visit Philemon. In fact he asked Philemon to prepare him a lodging. Again it is quite unlikely that he would be able to visit him any time soon, while being a prisoner in Rome. He seems to have been quite close to him.
And the peace of God, which passeth all understanding, shall keep your hearts and minds through Christ Jesus.