It is a great question and I agree with @DCGotiza that it’s a matter of context.
In order to get the most from a passage of scripture, we must seek to find the context and try to understand who the audience was and why, in particular, that topic is being addressed, especially in Paul’s letters to the churches. The point of Paul’s letters was typically to offer guidance in areas where they had perhaps taken a wrong path and provide encouragement for getting back on the right path.
Another point I will make is that Jesus and Paul, both use metaphorical language to build a word picture to illustrate a point. We see this many times in Jesus parables, and although many parables are/were shrouded in mystery and the point not clearly understood, the particular picture the parable painted was clearly understood (if I may use a metaphor of my own). When Jesus spoke of a gate, they knew the gate he was referring to. When he spoke of the sheep knowing the shepherd’s voice he knew the agrarian audience would clearly understand that a shepherd can enter a pen of hundreds of sheep, but only his will follow his voice.
When I am studying one of Paul’s letters, I almost always picture Paul like a lawyer in a courtroom trying to build a case for a particular truth. He knows that within the jury there are different types of persons from different backgrounds and so he uses a variety of arguments so each of these persons can understand the truth he is revealing and affirming. Often these arguments use metaphorical language that strikes a note of truth in his listener.
For example, we know that the church in Rome was a mix of Jewish believers (who had been in temporary exile from Rome and likely recently returned) and Gentile believers. From the tone of the letter Romans, there was likely some conflicts between the two groups concerning the proper way of following Christ. When reading Romans you can see passages that build an argument to appeal to those from a Jewish background, such as those that address the limitations of what the law can do. And passages that speak to the Gentile believers, such as “circumcision of the heart” (Romans chapter 2) and being “grafted” into the tree to receive the nourishment from the root (Romans chapter 11). Seeking to learn context can greatly aid understanding.
I think words like adoption and slave are both used to help us understand part of our condition and identity, but in different ways.
When scripture speaks of adoption it is legal language that denotes a change in status from not being in a family to belonging to a particular family. It is a similar kind of metaphor to passages that speak of being grafted into the tree, or a vine. Adoption means we have rights, privileges, and responsibilities that we previously did not have. It also emphasizes the chosen nature of our new status. We were adopted through the will and desire of the Father, the will and sacrifice of the Son, and the will and initiation of the Holy Spirit…freely. God was not obliged to adopt us. Adoption is a picture of grace.
Some years back, I heard a message on the Prodigal Son. When the Prodigal Son came home and the Father placed the family ring on his finger, that was a sign of adoption. It restored the legal status of son lost when he chose to leave the family and take his inheritance. That is a detail lost on our modern understanding, but context Jesus’ audience would have understood.
The slave language also is dependent upon context. @jlyons post concerning Mosaic law and bondservants is spot on for understanding how the first century Christian would have understood what was being described. But the context can vary, let me give a few examples. It can be in the context of understanding what we are bound to, chained to, or yoked with. Just like animals, human slaves were often yoked together in certain tasks. In Matt 11:29-30, Jesus tells us to take His yoke upon ourselves, that it is light and easy and will bring us rest. Or, in our bondage to sin, where we are incapable of breaking the chains ourselves.
The connotation can also be that which we are in submission to, where not submitting carries a penalty–often a heavy penalty. Another, and a context often seen in Paul’s letters, is that of being fully owned, or as we might say in modern language…all in. In a way, it is a metaphor for 100%…completeness.
I believe that the slave metaphor is used because of its extreme nature, it is very black and white in contrast. Contrasts like being fully-owned by Satan, or fully-owned by God…whose are you? Who do you identify with wholly?
So often the challenge with scripture is to understand when language is figurative and pointing us to an understanding, or to be taken literally. I know some believe all scripture is literal, but unless I turn into an actual sheep tonight, I’m not buying into that mindset. I rely on reference guides to help me understand the context for the original audience, then I seek to understand what is my context for today, for my own walk. We can so easily get things wrong, but I think more often we miss out on the depth that can come with understanding the picture more fully.
I hope something of this was helpful, Troy. It’s a great question and thinking about it has been a blessing to myself, and probably all who have posted.