Slavery was common in Ephesus; it was part of the social framework.  But the apostles were not commissioned to condemn the slave trade, any more than they were to criticise the Roman occupation.  Their work was to preach the crucified and resurrected Jesus (1 Corinthians 1:23).   This does not mean that Christians today are wrong to campaign against slavery, but the apostles' priority then was to advertise Jesus.  

Although some slaves had respected positions in wealthy households, slaves were considered to have no rights and some masters may have treated them badly.  Such slaves tended to grumble.  But when slaves became Christians, they were subject to a higher Master and grumbling had no place (Philippians 2:14).  Slaves who obeyed their masters in a godly way would be a massive witness to the grace of Christ (Ephesians 6:5-8).  The gospel would spread as whole households saw how the slaves' new belief changed their behaviour.  

So, how should slave and master relate when either or both of them came to know Jesus and join the church?  The worker should respect the boss, even the harsh master, for Jesus’ sake (1 Peter 2:18).  Grumbling in the workplace slanders God, who designed us to be good workers and to honour those who are over us.  If their master was also a believer, they should not expect special treatment nor should they show less respect because they are Christian brothers.  Those who serve Jesus should also serve their masters with even greater diligence.  Honouring God in the workplace starts with respect, obedience and hard work.
 
Many of these principles still have to be taught to new believers because they go against our own selfish default behaviour.  Without being taught the truth, a Christian can become a spiritual liability in the workplace especially where other believers are involved.  The family nature of Christian fellowship can allow the lazy, self-centred and ill-disciplined believer to spiritualise their faults and fail to see the significance of honouring God as they work.  So, let us examine ourselves.  Grumbling against our boss will slander the name of God.  However, if we do obey the Bible’s teaching, and urge others to join us in setting a godly example, the workplace can be transformed, and the Lord Jesus is glorified.