In a literal sense, this relationship does not exist in our part of the world (although it may exist elsewhere). Nevertheless there are principles for employees and employers in Paul’s words, which are not hard to work out. The focus on slaves here may have been caused by the slave Onesimus’ previously rebellious behaviour and Paul would not want other slaves to imitate him and run away.
From one point of view, slavery is the sad outcome of the old humanity, an expression of man’s callous indifference to his fellow humans. Can grace operate in such a disagreeable situation and can the principles of the new humanity work there? Paul’s answer is yes, to both questions. No matter how difficult, and at that time slaves would have no way of avoiding arduous demands from an unjust master, God’s grace could enable them to live for him there.
The only way to live in such a way is to put the Lord first and recognise that primarily one’s labour is done for him. There was probably a possibility of a slave in a Christian household, recognising the graciousness of his master, abusing his status by not performing his role. So a slave had to remember that the Lord saw the state of his heart and would know why and how his work was done. Jesus wants his people to serve him wholeheartedly, and the service of slaves was measured by its completeness, by its sincerity, and by its fervency. There was to be no uncompleted tasks or sullen expressions.
Furthermore, good work for an earthly master would bring no reward from him for his Christian slaves, but such work will bring a great reward from Jesus when he returns – a share in his inheritance. Failure to do such work, however, will result in loss of reward from God.
Masters are reminded that they too are accountable for the way they treat their slaves. The implication is that Jesus will judge any master who is not just and fair with his slaves. It would also be a bad witness if a Christian master ill-treated his slaves, even as it is a bad witness when a Christian employer does not gave a fair wage to his workers.
There was always the possibility that a Christian slave would have greater spiritual gifts than his Christian master. Perhaps the slave would be a better teacher. Should that be the case, the master should listen to what his slave had to say if he was explaining accurately the word of God. For masters and slaves to exist together in brotherly love would speak powerfully to a society torn apart by bitterness and dislike of one another.