I appeal to you for my child, Onesimus… 

(Philemon 10)

The apostle Paul writes while imprisoned; it is not certain whether in Caesarea or Rome. With him, as a companion for a time, was a slave belonging to Philemon, to whom the letter is written.  The letter is to be carried by the slave, Onesimus, along with another (the epistle known as Colossians) as Philemon is a overseer of the church Paul founded in Colossae.  In the personal note, Paul is asking that Philemon receive his slave with forgiveness:

… no longer as a slave but more than a slave, as a beloved brother – especially to me, but how much more to you, both in the flesh and in the Lord. (Ibid 19)

But to be clear, Paul does not ask for his freedom.  Onesimus is a slave and as Paul writes elsewhere, on becoming a Christian you should keep your worldly station.

Each one should remain in the condition in which he was called. Were you a slave when called? Do not be concerned about it. (But if you can gain your freedom, avail yourself of the opportunity.) For he who was called in the Lord as a slave is a freedman of the Lord. Likewise, he who was free when called is a slave of Christ.  (1 Corinthians 7:20-22)

Onesimus (Ὀνήσιμος) is a slave-name, oninemi means use or profit, in other words, Onesimus is a slave who is ‘useful’.  

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