5 Slaves, obey your earthly masters with fear and trembling, 6-7 with a sincere heart, as you would Christ, not by the way of eye-service, as people-pleasers, but as servants of Christ, doing the will of God from the heart, rendering service with a good will as to the Lord and not to man, 8 knowing that whatever good anyone does, this he will receive back from the Lord, whether he is a slave or free. 

9 Masters, do the same to them, and stop your threatening, knowing that He who is both their Master and yours is in heaven, and that there is no partiality with Him. Ephesians 6:5-9 (From the English Standard Version)

Masters and slaves were a big part of the culture to which the gospel message came. But the gospel didn't call slaves to revolt against society. Had it done so it would have led to multitudes of slaves being slaughtered by the military, and the authorities would've felt justified in attacking Christians for teaching revolution. Yet, what Paul and the other apostles taught, even about about slaves and masters, resulted in them being called “men who have turned the world upside down!” (Acts 17:6) What is an important word in vs 5 that a Christian slave could take to heart in the difficult role they had in society?

I'd say it's the word 'earthly'. They could take comfort in knowing that the situation they were in was a temporary one. They had a wonderful Master who was permanent, and who would richly reward their faithful service when the time came. (vs 8) That was the greater comfort they could take to heart to keep them going. With that in mind, how did he encourage them to go about their work? (vs 6 & 7)

He urged them (in vs 5) to have a sensible fear of their master. This was because their well being was very much in his hands. (Or ‘her hands’ if the owner was a woman). More than that, he urged them to go about their tasks conscientiously and to serve from the heart. They were to do that even when the master wasn't watching. That's quite a demanding standard. But it would be made easier if they remembered they were ultimately serving Christ. The earnest and honest way they did their work would be a witness to their master of how good their Saviour must be.

While slavery didn't come to an end straight away with the arrival of the gospel, it was through the influence of the gospel that slavery was eventually abolished in many countries. But there are still many jobs in the world that are looked upon as menial. They're often not highly paying, and the worker may still have a very difficult boss to work for. Yet the principle Paul set before slaves in the first century should still apply. We should always work in a way that brings honour to Jesus.

What would you say is a key phrase in vs 8 for those who were masters in Paul's day?

I'd say it's those words “there is no partiality with Him.” If slaves were called to serve an earthly master 'with fear and trembling', how much more ought those who were masters to serve their heavenly Master in that way! So Paul urges them to 'stop their threatening'. A modern word would be 'don't be bullies'. When soldiers came to the man named John, who was baptizing people in the River Jordan, and asked him what they should do to show repentance, John replied: “Don't extort money from anyone by threats or by false accusation.” (Luke 3:14) They were not to use their position of power to threaten and bully others.

The principle Paul set before those who owned slaves in his day is one that employers and bosses can put into action today. If the MD of a multi million pound corporation is a Christian, he or she must remember that a Christian janitor who looks after their Company building has the same Saviour and Lord as them! And He shows no partiality. So they must not 'lord it over' the lowly employee. No wonder people in those days accused Christians of turning their world upside down. Christians are called to discard the discrimination that's so rife. Whether we're a boss or a worker, we should serve Christ, and seek the reward He promises.