Why did Onesimus return to be a slave?

Why did Onesimus feel an obligation to return to be a slave?

Although the Bible does not give us a direct answer to that question we do have some clues to a possible answer. If we examine closely the epistle to Philemon, we find that:

1. Paul begins by reminding Philemon that he owes a debt to Jesus.

Philemon 1:8 Wherefore, though I might be much bold in Christ to enjoin thee that which is convenient,

2. Paul goes on to say that Onesimus was not a good slave to him.

Philemon 1:11 Which in time past was to thee unprofitable, but now profitable to thee and to me:

3. He also tells Philemon that he has brought Onesimus into the body of Crist and that he has since been of great help to Paul.

Philemon 1:10 I beseech thee for my son Onesimus, whom I have begotten in my bonds:

4. He also tells Philemon that he has sent Onesimus back to him, even though he desired to keep Onesimus to help him in place of Philemon himself.

Philemon 1:13 Whom I would have retained with me, that in thy stead he might have ministered unto me in the bonds of the gospel:

5. But adds that he would not keep Onesimus without Philemon's assent.

Philemon 1:14 But without thy mind would I do nothing; that thy benefit should not be as it were of necessity, but willingly.

6. Finally Paul lays a guilt trip on Philemon.

Philemon 1:19 I Paul have written it with mine own hand, I will repay it: albeit I do not say to thee how thou owest unto me even thine own self besides.

Paul also had most likely convinced Onesimus that it was his duty as a follower of Jesus to honor his master's ownership, as Paul indicates is the Christian thing to do in:

Colossians 3:17 And whatsoever ye do in word or deed, do all in the name of the Lord Jesus, giving thanks to God and the Father by him.

And further expressed in:

Colossians 3:22 through 25 Servants, obey in all things your masters according to the flesh; not with eyeservice, as menpleasers; but in singleness of heart, fearing God: 23 And whatsoever ye do, do it heartily, as to the Lord, and not unto men; 24 Knowing that of the Lord ye shall receive the reward of the inheritance: for ye serve the Lord Christ. 25 But he that doeth wrong shall receive for the wrong which he hath done: and there is no respect of persons.

Philemon must have acquiesced to Paul since:

Colossians 4:7 through 9 All my state shall Tychicus declare unto you, who is a beloved brother, and a faithful minister and fellowservant in the Lord: 8 Whom I have sent unto you for the same purpose, that he might know your estate, and comfort your hearts; 9 With Onesimus, a faithful and beloved brother, who is one of you. They shall make known unto you all things which are done here.

All Scripture is quoted from the King James translation.

Hope this helps.

You have to start by remembering that in the society of 50AD, slavery was considered an absolutely normal and proper part of society. To suggest in those days that slavery should be abolished would be the equivalent of proposing today that money and wealth should be abolished, or that eating meat was immoral. So your claim that "Onesimus broke a unjust law of humankind" would simply not have been understood by those of the time - certainly by non-Christians.

Secondly it has always been a principle that Christians should obey, except in exceptional circumstances, the law of the land. If the government says they should pay taxes, they should pay taxes. If the government says they should keep off the roads at night, they should keep off the roads at night. So in essence Onesimus has an obligation under the law to return to Philemon, and Paul sends him back to fulfil that obligation. 'Escaping' doesn't free him from that obligation, any more than getting away with a robbery entitles someone to keep the proceeds. Moreover, because Philemon is also a Christian, Onesimus is in effect stealing from a fellow-Christian if he doesn't return.

However there is much more to the book of Philemon than that. Paul makes a great deal of pointing out to Philemon the new relationship between him and Onesimus: as you put it, there being in Christ "no difference between human master and human slave". He strongly hints without ever expressly commanding it, that Philemon should free Onesimus because of this. He hints rather than commands because he realizes that Philemon is fully entitled to keep and punish Onesimus, and he wants Philemon to not do this of his own free will, not because he has been told to.

We will never of course be sure that Philemon followed through with this, but I personally like to think so.