Ok, I understand what you are saying here, but in Romans 6:15-23 the analogy is used not to teach we are more than slaves but rather adopted sons. It is used to warn what will happen if we go back to serving our former master of sin. This is what is suggested by v.15-16, "What then? Shall we sin because we are not under the law but under grace? God forbid. Do you not know that to whom you yield yourselves slaves to obey, his slaves you are to whom you obey; whether [as a slave] of sin unto death, or [as a slave] of obedience unto righteousness?" These sentences imply that we can indeed go back to serving sin, and as Peter said, those who do are like a dog returning to their vomit, or a pig to wallowing in the mud. So, too, is a man freed from enslavement to sin through Christ who returns to serving sin despite being freed from it. And in such a state, not serving the Lord who delivered him but returning to his former master who brought him only shame, his end will be to be paid by the master whom he was truly serving. I understand fully that if we sin we have an advocate with the Father, but at the same time Jesus is not called our Lord in name only. He is to be our Lord in reality, for if He is not, we have no rightful place in His kingdom.
Alright, let me put it this way: when we tell our children not to do something, like put their hands on the stove, or run across the street, do we just say 'don't'...or do we lay out what would happen should they do it? And does the information of what could happen make it surer that they children obey us? For example, kids, like sinful people, have a bent to want to do what we've just been told not to. However, when being told 'don't run across the road or else you'll be run over and die', that sort of lends weight to the place where they're standing...next to their parents on the footpath.
Paul starts Chapter 6 by telling us that we have died to sin, and walk in the newness of Christ. In 6:14 he proclaims that sin will have no dominion over us. With that explicit understanding, he then goes on in the beginning, and weaving throughout of our passage to spell out what the dangers of enslavement to sin looked like in our life before we were set free, the consequences it would have inevitably led to. But then, starting in Vv 17, he again reminds us that we were 'once' slaves to sin and have now become 'obedient'. It goes on, but there is a very clear 'once...now' symmetry within the passage. But if we read carefully, there is no, not once, intimation that Paul fears or expects, any of those he is speaking to to actually fall back into sin. Just like we do not expect our child, once he is old enough to fully understand WHAT will happen should they barrel out onto the street, to do so. The reminder remains fresh, as children get distracted, but no child, or person, acts in a way that places them in deaths path, not unless they have mental issues.
My point being, just because we see Paul listing 'once...now' in the passage...giving us information of what would have happened should we have continued in our enslavement to sin, does not necessarily mean he is voicing his expectation that Christians who do not 'obey' enough will, in fact, fall back into that enslavement. In fact, I would say that it's simply not there.
About the adoption analogy, I think Romans teaches something similar in Chapter 8:
12 Therefore, brethren, we are debtors, not to the flesh, to live after the flesh.
13 For if you live after the flesh, you shall die: but if through the Spirit you do mortify the deeds of the body, you shall live.
14 For as many as are led by the Spirit of God, they are the sons of God.
15 For you have not received the Spirit of bondage again to fear; but you have received the Spirit of adoption, whereby we cry, "Abba, Father."
16 The Spirit itself bears witness with our spirit that we are the children of God:
17 And if children, then heirs; heirs of God, and joint-heirs with Christ; if so be that we suffer with him, that we may be also glorified together.
Now, suppose the adopted son refuses to suffer with Him. Suppose he denies Him before men to avoid enduring persecution. Is he still counted worthy to be called a son? And maybe more to your argument, what about the adopted son who lives after the flesh and does NOT mortify the deeds of the body? As adopted sons we are called to conform to the image of our Father, and that does not mean physically but rather in how we conduct ourselves in our everyday affairs. Would we still be counted worthy to be called His true sons if we never did conform ourselves to His image, but went back to walking in the image of the evil one who formed our behavior before we were adopted?
Here we get into a really messy knotty ball. Because, who can really know? Let's say there a Christian mother over in North Korea. She is quietly faithful in a hard Country, secretly raising her small children to love Christ, doing the best she can in a terrifying dictatorship. But what if she is discovered, and is called on to denounce Christ or watch her children shot in the head. What if in that very second she stumbles out of fear for her kids and does it. And they are all killed as martyrs anyway. Is she any worse than Peter? Who are we to say that she made an unpardonable sin?
Or, what if someone was raised in a Christian home, felt they were a Christian, but married a man who abused her, and used the faith to do it. What if the Church never lifted a finger to help her and she 'turned away' from both Church and Christ for many, many years. So many of us would declare her either apostate or never saved at all. But all the while Christ resided as a small nugget in her heart, waiting, guiding, wooing. And in her 60's, something happens that opens her heart and mind and brings her back. Who are we to judge that in-between period and say she can't be adopted? Why can't she just be a prodigal? Why shouldn't we pray for her heart to be restored?
The fact of the matter is; we can never really know a person's heart or where they stand with Christ. Their fruit can often lead us to speculate, but even then we shouldn't stop praying that their adoption would be made sure. But the thing of it is this: any decent adoptive parent is not going to give up on a rebellious child, a hurting child, a prodigal child, a child that makes mistakes, a child that hurts - from their own sins or sins of another. If you are a parent, you know that that is not something that will ever stop being true. It may grieve you at times, tire you at times, anger you at times, make you broke at times. But they will always be
yours.
And after the purchasing price...I simply believe that Christ's promises to me is sure.