This is addressed to slaves (Ephesians 6:5), but it is also a great Bible principle for all.  Although the Apostle Paul never endorsed slavery, it was a useful illustration of his relationship to the Lord Jesus Christ.  He calls himself a servant (Greek: doulas = 'bond-slave') of Jesus (Romans 1:1; Titus 1:1) as do James the half-brother of Jesus (James 1:1), Peter (2 Peter 1:1) and Jude (Jude 1:1).  Many English translations use 'servant', but the original meaning is a slave who has no get-out clause.  The apostles were glad to be the Lord’s slave.  It was an honoured privilege to be chosen to serve Him.

But serving the Lord is worked out as we serve other people for His sake.  How were the slave masters to know the gospel?  By seeing the transformed heart of the slave which led to willingness instead of reluctance, and gladness instead of grumbling.  That attitude is caused by a passionate desire to be like our Master, Jesus – who was also a servant of the Father's for our sakes (Philippians 2:5-8).  Wholeheartedness is only stirred by love.  Duty can only go half way, love completes the course (Ephesians 4:15).  It may not be easy to love a harsh employer, but if we see our labour as pleasing the Lord we love, then we can even be joyful in tough circumstances (James 1:2) and thankful whatever our circumstances (1 Thessalonians 5:18).  

But God is not unjust; He will not forget your work and the love you have shown Him (Hebrews 6:10).  He rewards those who love and serve Him.  Although the Bible does not elaborate on the nature of the reward, its centre-point is God's personal expression of pleasure and delight - which is infinitely more treasurable than any civil or military awards.  Whatever joy we may receive in serving the Lord during our lives, it is His eternal reward that matters; and He will be exceedingly generous.  Jesus' words in Luke 6:35 are powerful reminders to serve with joy even when it seems unreasonable, "But love your enemies, do good to them, and lend to them without expecting to get anything back. Then your reward will be great, and you will be children of the Most High, because he is kind to the ungrateful and wicked."

Wise Christians go into every working day with the conviction that they are serving the Lord, and that they are primarily accountable to Him.  Nothing of value escapes His notice, even if nobody else can see it.  Even as employees, who can break our contracts (with the appropriate notice!), we are still bound to Jesus.  His covenant with us never expires and must always be honoured.  The key is in wholeheartedness.  It is not so much our duty to our contract but our love and devotion to our Lord, that keeps us bright for work each day.  And His reward far exceeds our salary or pay packet.