Romans 8:15 For you did not receive the spirit of slavery to fall back into fear, but you have received the Spirit of adoption as sons, by whom we cry, "Abba! Father!"
For those who are believers we are both bond servants, doulos, and we are also sons.
We often miss the NT idea of slavery because of our American view of what it looks like. In NT times a slave could become a slave under various circumstance; a POW, selling themselves into slavery because of insolvency, born into slavery and a few others. Most slaves didn't really have any identity, but they often were treated well, because their masters knew that the closer they were treated to their emancipated counterparts the harder they would work.
"Doulos is a Greek word that means bond-servant. In the New Testament times it was not uncommon for a freed slave to voluntarily re-enter servitude to a master whom he loved. This servitude was called bond-service. Only a freed slave could be a bond-servant, and once he had chosen to become one, he could never be freed again. Neither could he be bought or sold, and he served his chosen master until death. When a man chose bond-service, he was taken to the front entrance of the house, and his right ear was laid against the door post. A nail was driven through his ear, and a golden earring was placed in it. This golden earring was a symbol to all of the servant’s choice and of his immunity to being made merchandise. Paul and Peter spoke of themselves as being bond-servants to Christ. There is a service whose true name is freedom." Kemper Crabb
With that imagery in mind, should there be fear in serving our Master? If the idea of serving a human, sinful master could bring peace and comfort to a man, then how much less fear when we are subjecting ourselves to the Perfect Master. He gives us everything we need to complete the task He has for us, He knows exactly what task we will be able to perform and at the rate we are able. As this quote states, this type of "slavery" is really freedom.
But Paul didn't stop at bind slave, he went on to tell us that we are adopted as sons. This was also something that happened to slaves in those days. If the master had no sons of his own he could adopt one, or more, slaves and make them sons. They were no longer slaves and now would have all the rights and privileges of sons. This would have been a huge deal for the slave. Think about going from having nothing, except a master, to call your own, to having all that the master would give you as his son. From no identity to the master's identity. From no possessions to all his possessions. From no rights, to the same rights of the master.
This is us. We were slaves to sin and Satan and Christ bought us out of our slavery; not so we would live on in fear of going back our old master, but so we will live in newness of life as voluntary, free slaves to our new Master, enjoying all the rights and privileges of son-ship.
This should change the way we live. What greater privilege to be free from sin and in the service of our Perfect Master, and heirs of The Father.